To be honest, more than any particular ability I'd like to see more mechanic-inducing traits/styles, etc. and traits cross-classable to give hybrid-ish class use a turn, at least in non-endgame open world or specialized content -- deemphasize the particular abilities but retain the way they work through other means, making ability makeup a bit more malleable. Make the other differences more inherent to their damage and generation properties instead. [Simplify the way the particular classes are made (their particular potency values per ability and precise rotations) to something that does the same due to more core concepts rather than best spreadsheet outcome, such that additional (even player-unique) abilities could be added more seamlessly while also allowing for more diversity/"specs" in Jobs and classes (Aeris, Templar and Ravene Dragoons; Sky Seer and Battlerite Monks; Evoker, Iconic and Archonic Summoners; etc. via different unique key abilities/traits, the base skill sets/traits, Xclass abilities/traits, and gear choices.]
Same idea for jobs too, of course--more flexible ability makeup around central concepts. Bards = Songs; Ninja = Ninjutsu; Monks = Chakra; DRG = Jumps/DragonSoul; BLM = Advanced dark-wheel magic access; WHM = Advanced light-wheel magic access; SCH = advanced Ley magic access; SMN = Summons; PLD = Oaths/Boon; WAR = Defiance/Wrath; etc..
Yeah... Too many new abilities in my mind, and most of them using a slightly untraditional "spec" of their respective job... for me to throw just one ability out. : (