Cast time to 2.5 sec: How about 2 sec? This is more a practical consideration than anything. I get where you're going here, 1.5 sec is explicitly to allow oGCD weaving, and the objective here is to reduce oGCD weaving. That's good, but keep in mind the modern FFXIV encounter designs tend to require a LOT of movement. Slidecasting can only work so well, and a 2.5 sec cast MOSTLY removes the ability to slidecast effectively. While instant casts can be used in there, keep in mind that the only other Jobs that currently demand hardcasting, BLM and RDM, freely allow movement casting with various spells.
RDM has Dualcast but also has 2 charges of Acceleration (which has been reworked into a limited-Swiftcast-with-benefits - having not read into it when I jumped onto RDM the first time after EW went live, I was VERY confused considering RDM was one of my DPS Jobs of choice in ShB). Adding in Swiftcast, this means RDM can cast up to 3 spells with no DPS loss, 4 if it can lead in with a Dualcast (2 sec cast on the base spells of Jolt/Fire/Stone), and though it is a DPS loss, Enchanted Reprise exists as a movement tool (people tend to think of it as a "fix 100/100 mana", but that's a situation that should never happen to begin with and that's if you don't notice it has a 25 yalm range; it's a movement tool)
BLM can stock two casts of Triplecast, plus Swiftcast, which allows for 7 (?!) spells to be fired off on the move if necessary. It can also use Sharpcast (which stacks to 2) to generate a Firestarter or/and Thundercloud proc, or gain either one or both naturally as part of its rotation, 2 Polyglot stacks for Foul/Xenoglosy casts, and in a pinch has Scathe which it CAN use Sharpcast on to make it slightly less terrible. That means BLM can have as many as 11 (?!?!) instant cast spells IN A ROW in emergencies. And that's ignoring that additional Thundercloud procs could occur among the other casts for even more free movement. Yes, it takes a bit to generate all those resources, but the fact of the matter is, they CAN be generated. Oh, and we have to remember (because I almost forgot!) because of the way Astral Fire/Umbral Ice work, going from Fire to Ice phase also grants a free and instant cast Paradox. So we can add that to the list. Given rough estimates of Thundercloud procs and that you can have Firestarter and Thundercloud up at the same time, it's possible for BLM to relatively easily get 12-14 instant casts. And we also need to remember that Between the Lines and Aetherial Manipulation exist, giving BLM unprecedented movement for a Caster, allowing quick repositioning which can allow for more cast uptime/getting in a quick cast after using one of those tools before continuing movement. Now, I can't think of ANYTHING in the game that requires THAT MUCH movement, but I think people often forget all of this. People say BLM is immobile because they consider the lengthy casts not allowing easy slidecast movement through goodly length slidecast windows, but they forget just how many movement options, tools, and instant casts BLMs can technically string together at need, even if they do have CDs attached to them that limits their use. They obviously can't freecast the entire battle like SMNs can, but they're honestly more mobile than RDM without Enchanted Reprise, and are JUST as mobile if we all Scathe as comparable to Reprise (using their instant casts to ensure they don't lose Umbral/Astral...except wait, they can handle that with Ubral Soul and Transpose, can't they? So they have that covered, too!)
Honorable mention allows us to throw in SMN here, which has mostly instant casts, but has 3/4/5 (depending on spell speed build) hardcasts per minute. However, even there, it has Swiftcast to make any one of them an instant cast, and if it needs additional movement (e.g. during Ifrit after Swiftcasting the first), can use the once per minute Ruin 4 proc off of Energy Drain to get further movement. And that's when it's not in Titan, Bahamut, or Phoenix and has totally free movement, nor in Garuda where it has 4 1.5 sec GCDs worth of free movement.
The irony here is, WHM in its present state is already the least mobile Job in the game if you think about it. For all the talk of BLM being immobile, they have the tools for a LOT of movement (RDM is actually more immobile than BLM), and among the healers, I believe WHM has the fewest free movement tools. SCH has Ruin 2 at a DPS loss (but less than chain-casting Dia/Bio/Combust/EuDosis would be), but with no CD allowing for large amounts of movement and Expedience as a second Sprint for when the SCH and/or the party needs to move and Sprint is already on CD. SGE has two charges of Plegma and up to three of Toxicon before resorting to Swiftcast (and can cast barriers on the move), and has Icarus if any movement phase that forces them to get somewhere has any kind of stack where they can use it to quickly reposition and then get back to hardcasting. And AST has Lightspeed once per minute and a half, allowing 15 seconds or 6 free movement casts before needing to use Swiftcast. WHM has 3 Lilies IF it's sitting on overcapped Lilies (which you shouldn't do) and Misery for a total of 4 if you're playing suboptimally. (If we want to count Dia's natural refresh, then we have that same thing for all the other Healers, so it's a moot point, and likewise if we count Regen, AST has Aspected Benefic and Sage Eukrasia Diagnosis, so that's also a wash except for SCH, but again, SCH has infinite Ruin 2 and Expedience)
Which is to say, WHM is already the least mobile, most turrety of all the Healers as-is, and ARGUABLY (for short-to-medium periods of large amounts of movement, like Ex4's Phase 4 where she's just sending the clones all over while flinging the little AOEs, the ring AOEs, and so on) the single least mobile Job in the game. So how much less mobile should it be, considering that?
That is, I don't think people realize just HOW immobile WHM is compared to most other Jobs. People look at BLM having long cast times but forget BLMs can also go FFX Lulu Limit Break mode if they really wish to do so.
...sorry for the length of that, I just think it's important to explain by example what I mean instead of just say "WHM is already too immobile". I know some of you guys don't like my post length, so...I am trying to cut it down, but I also feel that if I just said it, people wouldn't believe me. This way, people will attack me try to poke holes in it anyway, but at least it's out there for reasonable people to go "Oh...yeah, he's kinda right..." Maybe...
That said: If you wanted to have some few powerful spells that are 2.5, that might not be so bad. Holy is already a 2.5 sec cast, and if Cure 3 was as well, that wouldn't be illogical, imo. I feel like it used to be, it and/or Medica spells...but maybe I'm misremembering. But it's more "filler" spell should probably not be more than 2 sec unless you plan to give it some serious movement tools like BLM has for 12-14 casts without having to stand still. Modern encounter design doesn't really allow any Job to NOT have such capability - and don't get me wrong, I feel like that's something in encounter design that's a bit weird (and part of why BLM is an outlier job in this game), but until that changes, it has to be accounted for, I guess?
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Water/Banish - Provided it's not near-melee range like Plegma, I honestly like this. Something we can use for movement that ISN'T a DPS loss when people are all topped up on health and we aren't in danger of Lily capping that can also be rapid fired if needed. Did you mean 20 sec CD or 20 sec refresh? I'm thinking Gauss Round/Ricochet from MCH, just as GCDs instead, was that what you're saying here? If so, then I'm in agreement with that. I like how Aerith doesn't have to stand and cast the little magic bolts she fires off continually and can just fling them out left and right rapidly if needed. Though it might not allow as much weaving, it'd be kind of cool to me if this ability had a 1.5 sec GCD (like Gardua SMN or Hot Shot Hypercharge MCH), but I get if it doesn't. (See? I'm not against some cool/fun DPS buttons when they aren't overcomplicating things or getting in the way...)
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Aero/Dia - I'm not really a fan of nature magics on WHM. To me, CNJ should have a second Job it picks up (like ACN does SMN and SCH) that's either Geomancer or Druid that really picks up that ball and runs with it. I feel like holding onto the elemental stuff is people clutching onto the past while WHM is supposed to grow beyond that. CNJs are acolytes of the Elementals and wield the Umbral Aspected elements (though oddly Aero instead of Ice?), but WHMs are masters of the Umbral aspect ITSELF, which we learned in our trip to the First is actually Light. I don't mind there being a spread mechanic of sorts for Dia, but the thing about it is, consider that right now, in dungeon runs, applying Dia to enemies while running and swapping to Holyspam when the Tank settles in having gathered everything into a clump IS our WHM dungeon gameplay. If you make Dia AOE, then that goes away. You could argue keeping that aspect with Aero, but why do we need TWO DoTs? You already know my feelings on one and my feelings on DoTs that aren't central to a Job's kit with interactions with it. What's the logic of having a second one when the second one is fire and forget, too? Or did you just mean for the spell line to go Aero -> Aero 2 -> Aero 3 (all single target) -> Dia (AOE) as a single spell line?
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My one issue with having to cast Holy - in addition to you now asking me to have a both a DoT AND an upkeep buff, knowing that I dislike BOTH of those things... - is that it ALSO is close range. Unless merely casting Holy applies the buff, but that causes cursed openers/reopeners (watched a Mr Happy video today about his proposed MCH reworks and he even notes that problem with his Flamethrower proposal and how it would have to be changed). One other solution to this would be to make Holy like Cure 3 where you can cast it on a target at range OR (if you have no target or an enemy [Cure 3] and in this case friendly [Holy] target) simply casts it at your location. That might be a solution and would also make Holy more distinct as it could be used like both Gravity OR like its current form, depending on the WHM's need. Honestly, that change ITSELF I think would be cool to have. And I agree with you that it's a cool (and iconic to White Mages in FF history) spell that is sadly relegated to such a side-role as "the AOE button".
I'm not opposed to this ENTIRELY, if Holy was changed to make it more usable. Though again, remember I dislike "busywork" spells, which I consider DoTs (that don't interact with your kit) and upkeep buffs (that also don't interact with your kit) to be. So this is yet more you are askign me to compromise on for what doesn't seem to have too much benefit...
Though I WOULD like a reason to cast Holy more often. If not an upkeep buff, something else. Here's an idea (just an off the top of my head proposal, not thought out, just spur of the moment), what if casting Holy generated Blood Lily? Not sure how useful it would be single target, but it'd at least make the AOE rotation more interesting since you'd be casting Misery more often in dungeons at least. Or possibly it could buff the next Dia or Banish. While I hate upkeep buffs or DoTs, I do like abilities that interact with other abilities.
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Lilies: I feel like Misery at level 30 might be a bit...much. I DO believe that Solace should be lower. I think I once proposed level 20 or so. It might be worthwhile to have it generate something like Misery but a lower tier version that upgrades later. I dunno, visually and stylistically, it seems it should come later because it's such a powerful thing. It's core to their rotation, but if BLM's got Xenoglosy at level 30, people would probably raise some eyebrows.
For spenders, that's an interesting idea. The one thing is we run the risk of homogenization - that sounds a lot like Protraction/Krasis, and we also don't want to accidentally make Lilies into Aetherflow 3.0, I wager we can agree on that? I've been saying for a while I'd like a Lily Regen. Two things are ALMOST never wastes - Regens and Barriers. You can throw one on the MT/4 man dungeon Tank at basically any time and it will get at least SOME value. Side story (sorry...):
In ShB, when Energy Drain was removed from SCH, I was among the voices complaining about an issue that it created. "So what do I do if I have 1 AF stack still, I'm coming up on the Aetherflow CD and need to hit it to keep the CD rolling, but Excogitation and Sacred Soil are both on CD?" Where many in the community said "This is why we need Energy Drain back so we can do some damage (and I guess get some MP...but damage, man!)", I had a different position in line with what you can tell is my general philosophy:
"So...WHM has Divine Benison. A non-barrier healer (we had the pure/barrier dichotomy then, AST just played for both teams and we all knew it) has that. Why does SCH not have something like that? Why does SCH, the OG barrier healer, have its main mechanic, Aetherflow, with no barrier abilities on it? Why don't we get a 300-400 potency pure barrier OR have Lustrate gain a barrier with a Trait at some point?" (At least then Lustrate wouldn't be near-useless...)
To this day, I still don't understand why SCH, THE barrier healer, has no instant cast barriers and no barriers tied to Aetherflow. Really stop and think about that for a minute. It is passing strange, is it not? But the reason I thought that was - barriers on Tanks are never wasted except during long downtime (when Energy Drain would be useless as well anyway)
Anyway, back to here. I love Regens. I loved playing a Resto Druid in WoW. I'm liking this tier's normal fights (haven't gone into Savage yet) because I have some cases where using Regen is actually good play (at least, to my way of thinking), but it's still a DPS loss (which feelsbadman). So either having Regen generate a Blood Lily itself OR having Regen as a Lily ability OR having a Lily ability that applied a Regen would be interesting, imo. Something to make it more generally useful and not bad play. In most MMOs (and even single player RPGs), Regens are valuable and powerful spells that you WANT to use and keep up on at least your main tank. FFXIV having pushed Regens to the wasteland of "Bad Gameplay" is kind of sad and it'd be nice if that was fixed. And as I say, Regens are RARELY wastes.
If we had "Afflatus Regen" right now, it wouldn't step on any toes and would give us a Lily ability that's always worth casting to prevent overcapping, and often worth casting other times.
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Misery being stackable with a 20 sec CD is honestly a pretty good move I'd agree with. Can't fit two in a burst window, but this would actually reduce the chance of overcapping, making WHM more flexible (and lowering the skill ceiling in that sense, but I'm obviously fine with that), so all around I think that's a good move.
Kind of disagree on the MP to Blood Lily thing - nightmares of WoW Warlock Lifetap for some reason. XD I already hate using Seraph Strike in Delubrium for this reason. Granted, you CAN Thin Air that.
Besides, in a world where Aetherflow and Rhizomata exist...
(Though I'll note here I'm starting to get worried about adding a lot of buttons, but I'll try and remember to come back to that...later...god this is getting long. XD)
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Arcane Ward...is an interesting proposal. 120 or 180 seconds is a long time, though. I actually like PoM as it is and it feels about the right time to me. I kind of hate the 2 min burst thing, but I have it in a nice spot where I can easily see if its coming off CD to smash it again, and it feels good when I use it...and doesn't require rooting.

Now, if we're talking about this as a super-powered PoM+Divine Seal, basically, for a level 100/7.0 capstone...I would not be opposed to that. Though I wish abilities didn't have CDs longer than 120. If I use Lilybell, the entire rest of the fight it feels like the damn thing is still on CD, while abilities like Aquaveil and Tetra feel just about right, and Asylum feels a BIT long, where it's SLIGHTLY uncomfortable wanting it to be up again sometimes, but not so long that it's TOO uncomfortable.
I know it's subjective, but I can't describe it better than that. 30 sec CDs feel spammable, 60 sec CDs feel good, 90 sec CDs feel slightly awkward, 120 sec CDs feel very "use on CD", and anything longer feels like either "I'm never going to remember/use this" OR "I used this and now it's never coming back again." XD I have no idea how better to explain it, though.
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Tempest is...interesting. So undo everything I said above about the second Misery charge being good and making the job more accessible. XD
That said...I don't hate this. Does the buff last indefinitely, or...?
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Button Bloat:
My personal philosophy is at LEAST one Job in each Role should have no more than 29 hotkey spaces. There's a specific reason for this, which is 29 + Sprint + LB + Potion is 32, which is the most you can fit on two sets of controller hotbars (any more and you have to start getting creative with the press and hold shenanigans), but honestly, I've never seen a good argument for "every class in an MMO should have more than 30 buttons". 30 is a LOT of buttons if you think about it. Right now, the Jobs in the game with the least hotbar button requirements are MCH, BLM (go figure, eh?), RDM, SMN, WHM, and SGE. I believe every other Job requires 30 or more Hotbar spaces if you slot all abilities and WHM/SGE if you slot all theirs (this includes role actions).
NOTE WHAT THIS IS NOT:
This is not saying that every Job should have less. It's saying at least ONE in each role should have less (more having less is fine, but if PLD wants to be hyper special and have 34 - the worst in the game in terms of hotbar economy - or SCH and AST 33, that's fine as long as there are some that do not). In any case, there's only so many you can add before we start running into that problem.
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As far as my headspace:
I just like casting heals. I don't derive joy from casting DPS spells IN GENERAL (Primal Rend, Double Down, Misery, and oddly Scattergun are really my only exceptions thus far). But I also understand DPS being valuable in a game sense, which is why I think the Devs (accidentally, probably, but a happy accident) stumbling on Lilies/Misery was such a good thing. It lets WHM leverage the design space of using GCDs to heal instead of always Glarespam (which I think we agree is a positive) since it can refund that with damage, and with a damage button that isn't obtrusive but is FUN to press.
That they stumbled on it by accident is irrelevant, that it's a cool stroke of genius is what's relevant.
I think if WHM design can really leverage into that more, it would be an improvement overall, though with the caveat that we also don't want Lilies to just become glorified Aetherflow like how Addersgall is. So additional Lily spenders, if any, should move away from SCH-like abilities - which is one reason I'm a fan of a HoT, besides the other reasons I stated above. The nice thing is, there's no ironclad rule saying what has to generate Blood Lily, meaning if other GCD heals did so, there wouldn't be any inherent issue as long as the GCD trade/damage neutrality was more or less maintained in a way that still seems balanced.