Blitzball and Bliztball tournaments.
NPC interations [Gain friendship points.]
Instead of the same sh!tty modes of PvP, implement this:
https://en.wikipedia.org/wiki/Fantasy_Earth:_Zero
Some sort of FATE type of thing for gathering. - Fun events where you make stuff for people like a wedding for an NPC.
Raids that feels bigger than what they are so more 24 man if not bigger raids.
Hard more dungeons that's actually hard.
I would say open world instances but this games combat kinda sucks compared to TERA, so open world instances and stuff won't be as fun as that game.
Retainer/Chocobo storyline/dungeon or something Idk, more interactions with your retainer and Chocobo?
Housing could be fixed...