Yes, you highlighted what I was eluding to with my statement about not wanting to use said skills during AF, which makes AF phase bland.
Simply removing the choice around using them, doesn't really solve the problem, it merely shifts it over to UI phase, since given you spend the majority of your time in AF it's likely that you will have Foul and Thundercloud being used then if it was not possible to hold them for any length of time (Which would be tricky to actually make happen, given that time in AF is limited by MP, if the case was that you couldn't hold onto these skills at all and they happened to come up after your final F4 so you're out of MP and time on AF you'd be forced to either skip them or drop Enochian... Not exactly fun gameplay)
It would be a better solution though, because given how short UI phases are, it's harder for the staleness to kick in much (Well... It's basically Thunder + Blizzard IV + Fire III if you're not squeezing a Foul in there somewhere)
It's not moot.
As is the case at the moment, Fire IV is working on an effective 540 potency, this causes it to be the single highest damage skill that is readily available. If anything was to compete with this potency, it would need to be designed in a way where we wouldn't just replace parts of the UI phase with it, so as to allow us to maximize our output within AF using Fire spells to make use of the potency boost it gives.
Making a new Fire aspected skill is the easiest way to make something only useful in AF phase.
True, though, you'd have to question the reasoning behind such an addendum.
Like, Fire IV and Blizzard IV having the AF/UI lock addendum makes sense because it's literally suggesting "You need to be in tune with Fire/Ice to be able to use these powerful Fire/Ice spells"
Having an Astral Fire lock on say, a Dark aspected spell would be odd.
What's convoluted about Penumbral Hearts?
We have Umbral Ice (Umbral referring to the Moon blocking the light from the Sun causing an eclipse). We have Astral Fire (Astral referring to stars, such as the Sun).
Then we have Umbral Hearts as a resource to use. This implies a possible "Astral Heart" to have as a second resource to use in a similar manner.
With BLM also being focused around a third element, Lightning. Which currently has no phase associated with it. Thus, I thought up the idea of "Penumbral Hearts" (Penumral referring to when the Earth is blocking part of the light from the Sun leaving the Moon in the outer part of its shadow (Which is called the Penumbra) the effect of which causes a reddish hue on the Moon which is oft referred to as a "Blood Moon")
The name still follows the theme of celestial bodies, without being directly associated with the Sun or the Moon and referring instead to the Earth (Meaning that in this trifecta of celestial bodies, we have Astral being the Sun, Umbral being the Moon and Penumbral being the Earth)
No we don't.
You cycle back to that because you're not reading my response to it.
There isn't a definitive answer for "How many Fire IV's is too many?" because that's not even the question that should be being asked. Since the issue isn't the number of Fire IV's that are being cast - Literally you could triple the damage and MP cost of Fire IV and reduce the rotation down to Fire IV > Fire I > Fire IV and it would have the same issue.
The issue being that, Fire IV is our Astral Fire phase filler spell. It's the spell we default back to when we have nothing better to cast. Just like how every other non-combo based job has their own filler skills (Healers have Broil/Malefic/Stone, RDM has Jolt/Impact, SMN has Ruin, BRD has Heavy Shot, BLU has Glower)
But... We simply don't have anything better to cast. Not in Astral Fire phase. Yes, we have stronger skills - Thundercloud proc and Foul have higher potency. But their lenience in when to be cast, combined with the complete lack of any actually worthwhile Ice skills, means that it's never necessary to cast them during Astral Fire phase.
If there was a conditionally better skill to use during AF (Such as how at levels 42-60 we have Firestarter procs to allow us to use a free, instant cast Fire III) or if there was actually worthwhile skills in UI (So that "Saving" TCloud and Foul to use during the time we're actually in that phase wouldn't be a thing) then there wouldn't be this issue of staleness with AF and the Fire IV spam.
The thing is, that neither of the above points are easily solved. The former solution, whereby there is a conditionally better skill to use during AF, is plagued by the 2 main design pitfalls for it:
1) There's the possibility to add in Firestarter type RNG. But people tend to not like DPS being so directly linked to RNG skills.
2) There's the possibility to add in a new resource that can be generated and then expended during AF to utilize a new skill. This has the problem that it can be too consistent. Consistency will cause any more potent skill to simply be slotted in to an appropriate place within the static AF rotation, thus not actually achieving any sense of "Shaking up" the rotation.
The latter solution, requires a rework of core systems for BLM, whereby the entire design of AF/UI gets upended, so that there isn't the dichotomy between "Damage phase" and "Procs phase" and instead it's more of "Damage phase" and "Other damage phase" - With the differences and nuances between them being about resource management, both with MP and potentially other resources that can be expended during UI and generated in AF. Which, if designed well, could also allow for more dynamic gameplay, wherein rotations are less about doing static rotations during predetermined periods of time in each phase and more about adapting to the situation such that maybe you swap between phases more frequently to juggle resources instead of "Bottom out MP > Swap > Regen all MP > Swap"
Now... As a realist, the most likely scenario is that they will take the initial solution and just stick an RNG proc into Astral Fire. Since, that's what they have fueling the other 2 real mages - SMN having Ruin IV procs off pet actions and RDM having Verfire/Verstone procs off their Verthunder/Veraero spells.
Though, while it's certainly the simplest and most likely solution, as a player and forum user, I would like to discuss the concept of "Is it the best solution?", which would include, thinking up potential ways to create alternate solutions and having them assessed by fellow players whom enjoy playing the class - Since any solution should be aimed at making sure that the players of the class will still enjoy playing the class and ideally will get more enjoyment out of playing the class.
Thus, I come up with things that can maybe generate ideas. Since, more ideas put together allows people to take the best parts of multiple ideas and then refine them into the "Best solution"
Is sticking a bunch of Astral and Penumbral Hearts onto the Job gauge and calling it a day the "Best solution"? No, obviously not. But maybe such a thing might provoke someone into having an epiphany and formulating an idea from such a premise.
Like... Maybe Astral Fire phases utilize Fire and Lightning spells to generate Astral Hearts and Penumbral Hearts. Which are then used during Umbral Ice phases to empower Ice and Lightning spells to a competitive level. Creating the aforementioned "Damage Phase" + "Other Damage Phase" style gameplay, with an emphasis on resource management between both phases. - Potentially with such a gameplay shift altering other parts of the job gauge. Like... What if phases no longer had durations or stacks and simply lasted until we swapped them? What if the only timer we managed was Enochian? What if Enochian, instead of simply granting a Polyglot, also put us into a temporary "Penumbral Lightning" phase which had some interesting and cool effect to it that maybe upgraded some skills or something (Maybe this phase is activated using a Polyglot instead of using Foul and wants to be timed for when we have certain CD's available or certain resources generated)
Maybe something else entirely?
I don't know. I'm only one person. There are only a finite number of ideas I can generate in the time I spend thinking about this sort of thing (I.e. The time it takes me to type out each paragraph >.>)