List of rarely used abilities for the benefit of SE

[Mithent]

BLM: Apocatastasis

This ability increases one party member other than the caster's resistance to fire, ice and lightning by 30% for 12 seconds, has a 120 second cooldown, and places a debuff preventing reuse on any individual for 60 seconds. It's only of any use if you know a big fire, ice or lightning ability is going to hit a single target other than yourself pretty soon. The game doesn't always make it very clear which attacks are elemental, but I don't think that most big hits directed at tanks fall into this category (e.g. I don't believe that Twintania's Death Sentence is fire-based), and being able to reduce damage to just one person from certain AoEs is rather situational. The duration isn't very long so it's not very useful to use it speculatively. I've used it in WP when I know one individual might be targeted for Fire, but it doesn't see a lot of use.

I'm not sure in what direction I'd take it to make it more generally useful but not too overpowered. Expanding the protection to all elements would make it applicable to most battles, rather than just ones which happen to use those particular elements (although they are linked to the THM/BLM spells, I know). Perhaps the duration could be increased and the protection changed to be 30% resistance to the next elemental attack or two in a manner that's similar to Manawall? Or the effect could be weakened but apply to an area around the target?

BLM: Freeze

I agree with the OP that Freeze isn't useful enough. The damage is too low for it to be considered by any AoE rotations, and the bind is rarely useful when we have a superior AoE CC in Sleep. Freeze has the same cast time as the WHM Holy, but has a smaller radius, half the potency, and a weak CC (enemies can still cast and perform ranged attacks) with a longer duration versus a good CC (Stun) with a short one. Freeze does have a few advantages: the CC is longer, it's endlessly spammable due to Umbral Ice, and it's targeted. But the first two are near-irrelevant, since BLMs already have another CC that actually stops attacks, and you wouldn't want to spam it as the DPS would be very poor compared to other alternatives. I agree with the OP's suggestions: granting Umbral Ice III or a potency boost would help to get it into AoE rotations, and a better secondary effect could make it more situationally useful.