Hmm, well in terms of gameplay, I would like to see a heavily updated Sphere Grid system. Yes, I liked the Sphere Grid. Come at me, bro.
I also would like to explore all of the islands and leftover parts in Spira that were never used in the previous games. Come on, you know you want to find out what's on that island near Mt. Gagazet, or what's on that long strip of land near the Thunder Plains. Also, a pretty minor thing, but what about an airship world map interface, only it's 3D and you can move the airship around like Kingdom Hearts II's Gummi Ship overworld?
If they somehow get dresspheres in there, how about an option where you can make your own custom dressphere design? Like SPORE, where you dress up the character on this interface using armour and weapons you find along the journey. Also, you can add general stat bonuses if you complete a certain armour set on a dressphere design. Plus, the playable characters have their own exclusive armour and weapon sets.
For example, removing Tidus' head will make him invincible!