There is ALOT of content in this game already if you really think about it. They could just enhance certain things and improve upon them. Here's a little list of mine:

Companion system (more companions, armor, abilities, fighting styles?)
Retainers (The possibilities here are endless, there could even be a big questline you can do with whomever you choose)
Pet classes (Why are there no more pets with diverse abilities?)
Seige (For anyone who's played FFXI, aht urghan under seige by invaders. The same could happen with ishgard and dragons)
Roaming Huge Monsters with "totem" rewards (similar to Odin, Behemoth)
Work on Housing situation and expand on it
PVP (I don't do PVP but their are alot of different kinds of PVP and their really isn't much here)
Materia system needs work, its either not used at all or becomes completely necessary (Why no crafting materia?)
Fates, Why not have some crafting or gathering fates to encourage people to try it?
Make everything DFable, please. Not everyone has time for statics or works third shift like me.
More interesting story quests similar to Hildibrand (does not need to be nearly as long or have to have the same characters, light hearted comedy every now and then is fun)

Just a few things off the top of my head, I'm sure there's more.