So, first of all, Soulsow is legitimately a garbage ability. This is the third time this garbage piece of crap has been added to the game. The first time was Chakra in Heavensward. Second time was Shoha in ShB, third time is Soulsow. What makes it garbage? You only can use it during downtime, meaning it is a button that exists on your bar that, in any non-ultimate fight, gives you a best case and staggering three uses of the ability. Effectively and often giving it a 5+ minute implied cooldown on that type of fight. A fight with 0 downtime gives you one use and it's gone. To even bring up this garbage bin excuse of an ability grossly misunderstands the point I was making as well as the design of that ability.
Second of all, I'm not even talking about downtime options here, but rather what mechanics should be afforded a downtime option. That is, I don't care about meditate giving you rage bar or stacks towards shoha. But rather meditate being used to extend your personal buffs, as you directly posited. Downtime building is fine. Classes like BLM (assuming it's not a cutscene) can generate xenoglossy, while classes like SAM can meditate. That's fine, it's not even part of my discussion or thought. Though if we're being critical for the sake of it, for the same reason that Soulsow being effectively a 5+ minute cooldown, buttons like meditate probably shouldn't need to exist anyways because it's an entire button slot when some classes literally don't have to do anything to get the same benefit. This isn't about job disparity, it's about efficient use of button space to either declutter classes with bar bloat problem.
Yes, exactly, every button should be important. When you can go entire fights without using a button, or barely using it, that button better slay god or the devs should consider outright removing it. A perfect example is something like Huraijin or Soulsow, which may as well not exist if a fight has no downtime. A weird example would be Transpose and Umbral Soul. Though both can be used in incredibly gimmicky rotations by exploiting BLM's generally awful structuring, but that's another topic unto itself.
Personally, I think Heavensward ninja was the best Ninja ever was, and everything after it is a step backwards. Too much business, not enough substance. It had 1 major flaw (ping dependency,) but otherwise was smooth. But as the class has definitely moved away, it slowly just destroyed the original thing. However, on that second point, let's look at it.
What might we do with Huton? First, we should look at the obvious. Huton represents the only combo in the entire game that is never used in combat, that isn't also fuma shuriken itself, which is kind of used on TCJ anyways. There are only 7 possible mudra combos at the moment. X, X-1, X-2, X-3, X-X-1, X-X-2, X-X-3. The devs went out of their way to make sure the garbage bin option, X-3, was used in Shadowbringers for multi-shuriken shenanigans. So the most obvious thing would be to look at the class and try to make the X-X-1 combo used somewhere else. So, what might we do, where might we do it? If we really want to take a step back in time, we can look at ARR's ninja design. Your 1-2-3 melee combo had no positionals. Cool, let's make Ninja a positionless class. Not because we need to, but to add some real choice with ninjas. We don't need to, it could also just be a 24/7 backstab simulator as well. We can get rid of armor crush, as well, since under this supposition, we're looking at a ninja that just permanently has huton active, like monk's Greased Lightning.
From this framework, what might we do? Well, the most obvious location is looking at the skill TCJ itself. Meisui exists to give TCJ's suiton a use. To spend it. So if we no longer make TCJ give you a suiton, we can remove Meisui, a button that may as well press itself after you cast TCJ. So what is this button doing? We could do a 4-step mudra combo. It was teased all the way back in stormblood. At level 72 or so, give a trait to upgrade TCJ, and make it so that your 3-2-1 combo makes your next mudra used do an empowered effect. 3-2-1-3 would be some slaying god tier single target attack, 3-2-1-2 would be a doton that looks like an exploding volcano or something. 3-2-1-1 gives you mega-bunny, which is a DPS loss disaster recovery option where your bunny whacks the boss, in case you mess up, to make sure it's not a critical damage loss. If the fourth step did 2000 potency, the megabunny would do 1500.
By doing this, you can potentially reclaim 3 bar spaces (meisui, armor crush, huraijin,) 1 dead mudra space (huton->empowered four step mudras,) and expand on the theme of the class in an exciting way (4 step mudras.) This is more what I'm getting at. I'm not saying this needs to be done. I'm not even strictly saying it should be done exactly like this. But rather that there's a lot of real estate tied up in bad abilities to facilitate this design that literally dates back to ARR.
So, AF/UI was never truly a fine mechanic. It's a very interesting study in bad design that has just been stretched and mutated into oblivion. The class was designed around a concept. Press X button, it stacks a thing, you do more damage. You run out of mana, press Y button, it rapidly regenerates mana at the cost of per-second damage during that moment. It was always designed this way. Transpose was a mostly dead skill used to recover mana after a fight ended or if a boss jumped, because you were forced to lose AF/UI timers during this downtime. It's a tale as old as time within ARR job design.
The difference is, for AF/UI, the devs decided to add fire 4 in heavensward. AF/UI was turned into a structuring mechanic off of a mechanic that was designed to drop and designed to basically have permanent uptime during a boss fight. The class was originally designed without AF/UI structuring the mechanic, and it was shoehorned into the class as a structuring mechanic. If we deep dive the mechanic itself, there are a ton of abilities that this weird pseudo-structure requires to maintain itself. Fire 1, used about once every 30-40 seconds, to refresh the timers (now done with paradox.) Fire 3, used for procs, which are also a core class mechanic that is about as vestigial as a dog's dewclaw by this point in the class design, but is used once or twice every 30-40 seconds. Blizzard 3, which is used once every 30-40 seconds. Transpose, to get back into mana generation, and umbral soul, to maintain stacks during downtime. This is specifically ignoring non-standard rotations. All of those abilities are required to maintain the timers. F3, B3, F1, B1(lul,) Transpose, Umbral Soul. All of those abilities are used, in a standard rotation without procs for fire 3, exactly once every 30-40 seconds, if not less than even that. This is before factoring in B4 (once every 30-40s), Thunder 3 (once-ish every 30-40 seconds, once every 30 is ideal,) and a whole host of other abilities on the class that are basically: "Use once every 30 seconds." Now, given the class is mostly fire 4 spam, some of these use once every 30s are whatever and add nice texture to what would be a boring class. But when the class literally has as many buttons as Samurai, there is legitimately an argument something is wrong here.
So, considering that AF/UI represents a massive amount of BLM's skill floor, the mechanic was shoehorned in as a structural mechanic, and it eats up so many buttons just for the maintenance of the ability, it perfectly encapsulates the problem I'm trying to underscore here. An old mechanic dating back 5 years (10 in the case of BLM,) that is eating up a ton of bar space and bespoke resources. At a certain point, you need to look at the class design and think: Why the hell is this mechanic here?
Honestly, Bard's DoTs have never been fine. Or rather, more precisely, DoTs are a bad game mechanic outright. To expound on this particular point, what is a DoT? DoTs are a fixed amount of damage with less crit variance that take a long time to do their damage. So, what makes a DoT interesting? By itself, nothing. It is boring and bad damage. It can act to shake up an otherwise stale rotation, but that is about all it actually does. A DoT ticking for damage isn't exciting. There's rarely animations for ticks. You almost never see the damage yourself beyond the initial and sometimes awesome animation (Scourge, my beloved!) Aside from that, the DoT itself is boring. Unless you're world of warcraft. In WoW, you can extend DoTs. DoTs can tick faster with more haste. You can use buttons to cause DoTs to tick twice as often, or to globally extend DoTs much like Summoner's old Contagion. When the class is less tied to a burst meta, you can have options like this. WoW also has animations for DoTs I think, and can make the act of watching DoTs tick interesting.
Unfortunately, FFXIV has none of these, so DoTs are just delayed damage. The best-designed DoTs are Warrior's Storm's Eye and Reaper's Of Death. And I'm only tolerant of WAR because the class has nothing otherwise, whereas Reaper's could be removed and the class wouldn't lose anything meaningful.
It's 3 buttons for a class that is primarily designed around RNG mechanics. It stands out as just a weird choice. It was fine in ARR and even HW when Bard was a vastly simpler class, but modern Bard where it takes 45 seconds for a mere combined 750 potency, of which you will probably clip 5 seconds or 2 ticks of it without top tier play, it strikes as a largely orphaned mechanic at this point.
BotD is more one of those things where in Stormblood, you could, and did, lose stacks due to boss downtime. They had to make mechanics to extend it. Also it was nuanced even in Stormblood, where you had to reapply BotD before using LotD as the timer was directly tied to how much time you had left before bursting.
If you follow the 'natural,' rotation, that is you use the class and push buttons on cooldown without saving things, ideally in the highest to lowest potency order, then if you try using perfect balance on a purple combo after you did a bootshine-true strike-anything, you will go into perfect balance, and during your burst, Disciplined Fist will fall off before you can fire off your purple combo for elixir field. We can argue whether this is a good or a bad thing, but considering it is a skill floor issue, and not a skill ceiling one, it shouldn't behave this way. Considering the devs have often smoothed this out for most classes, it definitely is a problem.
So, on the topic of abilities like that, I look at it another way. If you use X oGCD, and immediately follow up with Y oGCD, there's a problem. I'll use machinist for my example.
You want to Barrel Stabilizer, and then wildfire, and then hypercharge. It's not strictly always this exact order due to heat overcapping and people thinking 5 heat is a problem, but it's there. That's 3 buttons, 2 of which are dedicated 2m burst on top of yet-more 2m burst (Queen, mostly,) and is a very odd choice. Wildfire is just: "Better hypercharge." Literally. That's all it is. So if we take Wildfire, and just automatically launch you into overcharge, it's already better as is. If we take Barrel Stabilizer, and use it to upgrade Wildfire by, say, changing Heat Blast into a different ability, we can combine 2 buttons into 1 and delete another oGCD from the rotation by skipping the middle man. We can even ignore giving heat because Wildfire just straight up can put you into hypercharge.
For SAM, I'd take Guren and Senei, upgrade Guren into Senei but make them both hybrid ST/AoE skills. Then take Ikishoten, upgrade it into Guren, upgrade it into Senei, and just reduce the kenki it gives you by 25 due to using an empowered attack. I would then take that combined mess of 3 buttons and have it, when used, it turns into ogi namakure. By doing this, you delete exactly 1 oGCD from your 2m burst for literally the same damage and 3 less buttons bloating your bars, while also dropping the skill floor and, strangely enough, raising the skill ceiling, because you can actually store more kenki going into your 2m burst, adding some interesting, if not terribly important, burst potential to your 2m cycle. It's not a lot of extra damage, but anyone who truly wants to maximize the class would appreciate going into 2m burst with 60-75 kenki and being able to blast just that little bit harder. Whereas people like me can spend down and only miss party buffs for about a combined 750 potency of potentially stored damage.
I digress. There's a lot to digest here, so it takes a lot of time to go over.