Gunblades, maybe some corsair style class that exclusively uses them, but I just wanna see my favourite weapon of all time (although they've brought it back for FFXIII - thank you squeenix!)
Oh and the buster sword, masamune etc you know you want one in there. And while we're at it, not only the victory music upon finishing a fight, but give everyone a super-*** victory pose/dance lol. Even just a smug flourish of weapon spinning animation would be nice.
The main combat thing I'd like to see is actually by the gunblade but was far better realised in legend of dragoon - you know the one, that thing where you do button bashing in time to pull off awesome combos - if they could somehow incorporate that into group combat that would be amazing.
I think the outfits from some of the classic FF characters would be well cool as well (well ok maybe not vaan or anything) but still, be awesome if eventually melees could walk about dressed as Auron, Squall, Cloud or Sephiroth, or Assassins could get a Vincent outfit, Female black mages dressed up like lulu etc. There's enough characters in the series to still give a whole bunch of variety to the players.