Gameplay... maybe. 
I hope I'm wrong too. But we can only take a look at Squeenix's rep in the last few years, it hasn'y been rosey.
Besides, FFVII is technically an even bigger game than XV. Recreating all the assets from the ground up takes an inmense amount of time, even if they keep all the mechanics the same.

But but..... I couldn't wait any longer... If PS4 will be the first platform release, I don't mind getting it just for this game. I can tolerate not getting playstation for any games released except this!         

My worries now is.... Please don't make this game like FF13 battle system. I'll be damned seeing the battle system change. But I do hope all those mini games like battle arena, chocobo run, bikes racing are still retained as well as the fort condor defense game. Awwwww.... I think I better replay the game one more time, this will be my 78th times playing this game.       

FFXV hold ups over the years was caused by the game engine they've used.
I doubt they'll return to using it when KH3 is using UE4.

I'm one of those who never finished FF7 (played it until the submarine section I think). This will be a fresh experience for me.