Character/Class customization
I've read through many many threads of multiple issues of nearly every class and also threads complaining of bland classes and gameplay. I really feel there is one thing that can fix most of these concerns completely.
In a game especially an MMO the players want to connect with their character and feel like they are in a sense unique. Character customization is one what to help players feel this way and help immersion into the game itself as the player is more involved in how their character plays.
So far the only real customization is stat point allocations, which is reasonably moot since all points are generally allocated into the classes main stat. The only class this isn't as straightforward is acn being the only class with a dilemma of where to put points. This basically forces players to pick playing smn or sch but not both because one or both would need to be gimped, which alone is one of the more voiced issues about the class.
My recommendation would be to add a customization/specialization/talent system. I'm not saying something super complicated like Rift, but even a simple set of customization trees would alleviate most of the actual legitimate issues people have with this game.
A few examples would be to give summoners the chance to spend points to make a specific pet most effective or to improve the summoners own damage, depending on how the person wants the class to play. Or giving warriors trees that will either make them a more effective tank to match a paladin, or improve their damage so they can be full time OT and actually hold their own in DPS when not tanking. Or giving bard trees to make them even better dps, or provide more or better songs to increase party effectiveness, again depending on how the player wants their class to play.
The crazy thing is I half expected something like this to already be implemented due to SEs overwhelming need to innovate character progressing and leveling systems in their single player RPGs, and even in FFXI the sub-class option gave characters a decent amount of depth, even though it was similar to stat allocation as specific combinations were deemed the way to go.
This would not only fix peoples specific issue with the classes themselves but also provide easier access to class balancing as specialization trees are far easier to balance classes with then changing base coefficients or ability potency.
I really think this would make FFXIV a place to be and I personally know that it would keep me here for years to come. But I have to admit the lack of a system like this could be a major issue with a large portion of the MMO community and would eventually wear on people until they feel they are bored or the gameplay is just too bland, because they don't feel like they are a part of making their character unique.