I like all sorts of battle systems. There's some nice perks to the battle systems they used in the likes of 15 and 16, but I really do love turn based combat and perhaps what I absolutely miss the most is control over my party members. Heck, I wish we could even just have a gambit system like 12 so that I can at least control my party member's strategy, even if not them (though 12 also did let you take control of anyone -- just it was too tedious to do for anything other than infrequent moments).
Most specifically, I'd love to see a battle system like 10 return. It was one of my favourite for battle systems. Sure, ATB is nice and perhaps the most Final Fantasy, but the dynamic turns of 10 was very cool. I also really liked that it let me take time to think and plan out my moves. I could take a look at how each possible move would affect the upcoming turns. The whole turn order was super strategic. Being able to see when your enemies would attack was a big part of that (I don't recall any variant of ATB allowing that), because I could do stuff like "I have 3 turns before the enemy, so I can just press the attack instead of healing". Actually, I dunno why 10's battle system has never been used in any other game I've played (if you know any, lemme know!).