And on the opposite side of the coin, they have to make all classes simple enough that nobody is going to go "That is too obscure/complicated, I'm not going to touch it."
I would love to see a chemist-style healer, where your "moves" are basic ingredients or tools, and you mix and match them for different effects. I think along the lines of, your basic heal would be "Salve" and shortly after you get "Salve II." You also get a basic AoE, "Healing Rain," and an esuna, "Remedy." So you get the option to mix every 15s. You use mix, and you mix your "Salve" and "Remedy," and then you have an esuna that also heals a slight amount. Likewise, you use "Healing Rain" with "Remedy" and your AoE heal might not be as strong, but it esunas everybody in range. And then obviously as you level up you learn different recipes, effectively unlocking new moves, but keeps the amount of combinations specific enough that you don't have to memorize a huge list of abilities.
I also like the idea of a dancer controlling the positions on a battlefield somewhat. They could have styles, so that everyone within range of them gets a tp or mp regen. The range would be smaller than normal but more effective than most buffs we have. They could even create static AoEs, so make a bubble that lasts 20 sec and everyone inside of it gets a spell speed/skill speed boost. I also would like them to make a system where, if a dancer successfully dodges an attack, they get strength boost for 10 sec. Or even thinking along the lines of an AST, if they got a move that required them to become stationary but while in that stance, all heals on them spread to those around them, at a lowered potency. Just a class that can do DPS but is overall involved with other party members more, not so much like the design we have now where everyone is essentially a lone wolf in design, partied with other lone wolves. (And I mean this in a way where a dancer would be optimized and flourish in a party but would not struggle with solo quests and content.)
I think we can all agree on one thing though: I hope they never add more jobs based off of current classes, because they just won't diversify the jobs enough, and I worry that more jobs will become like SCH and SMN were in 2.x.