These are just some ideas I've had while talking with friends who play. Don't mind criticism, just keep it constructive and civil.(Note: I will edit this over time as new ides come my way so keep checking back. If you want to contribute, post it below and I'll work it in as soon as I can. )
-A glamour plate per class (And I do mean every class, including crafters and gatherers), perhaps with a couple extra for swim/civilian/nightwear (could even charge for them in the cash shop, I know a few people who'd pay)
-Blue mage queue system that works like normal (autofill) queues instead of party finder. If they add other limited classes letting them queue together would also be nice I think (though I realize that is, potentially, a big if).
-A means to gather mats while offline at a greatly reduced rate (gain class XP too), could also apply to crafters
-More control over what your retainers look for for you, not necessarily a guarantee but an increase chance of getting -item type here- possibly also including from -this level bracket- from any kind of venture, even the quick exploration.
-Level skips for crafters/gatherers, including mastery (another one I'd personally pay for...)
-I'm not entirely sure how to phrase this one so please bear with me... I'm up nights, and during most of my day there aren't enough people on to get queues for a lot of content without waiting for hours, if it ever happens. Perhaps have an A.I. option? Not sure what to call it apart from maybe an extension of the trust system, or maybe that crew of trainees you can get from the Grand Companies if their A.I. could be improved...Whatever we want to call it, some way to get into things like raids and challenges like The Great Hunt (EX) without having to try and solo them (I know, can't solo raids anyway, not the point) would be greatly helpful to some of us.
-Crossovers with Persona 5 and FF7? (I'd totally buy Vincent's outfit, and Joker's of course)
-A way to increase the size of your personal apartment
-A 'dressing room' function to allow for easier glamor work/planning