That's really interesting. I like the idea of encounters that give you more to do than "hit that until it stops moving." I'd be a little concerned about class exclusions, but given FFXIV's apparently-huge party size, that's probably not really that big a deal at least for event fights.
FFXI allowed you to have 18-man teams and the player-base still resorted to either "Throw more BLM at it" or "Throw more SAM at it". You can design mob fights all you want with the idea that the players 'would' want to use various jobs, but it hardly ever plays out that way. The player-base in general will always go the easiest route. If you design it where the players are forced to have or use x-job/ability in the battle, the players will just not do the event/battle if the reward isn't worth it.
SE designed several low-man instanced content with some really decent gear rewards over the years in FFXI. Yet, because the player-base in general felt they were too hard to figure out or setup stopped playing the content soon after it was released.
If you don't create jobs that are unique enough yet with some overlap in abilities/roles, you end up with a bunch of people in solo-mode. Might as well make a single-player console game at that point.