Besides Sephiroth, what Final Fantasy villain would you like to fight as a boss in future Kingdom Hearts games?

Tbh, I don't miss Sephiroth, and I don't actually care that much for FF cameo battles in general; I blame that long-winded sequence in FFIV: The After Years, and the fact that Gilgamesh is always relegated to the exact same jokes and a made up "bromance" with Bartz. lol

BUT, Ultimecia gets my vote. She would be so amazingly unexpected and out there, and unlike the others, she would actually get some visual changes to account for the E10+ rating. And all her magic usage could be pretty cool, graphically speaking. Honestly, the fight would probably feel out-of-place and surprisingly ominous for KH, considering she's actually really creepy (her up close attack would be scratching you with her claws) and FF8 is pretty trippy in general. A rearrangement of just about any of her tracks would be great, too. (I'd expect "The Extreme," but if they found a way to bring in the unsung hero of FF boss tracks "The Legendary Beast," it would be even more amazing.) And they can use her as an excuse to do something with Leon and bring in Rinoa, since it'd be a waste not to. lol Plus, an Ultimecia-themed Keyblade!

I'm gonna be a rebel and put my second vote for Barthandelus, if not just for the fact he'd also be super unexpected and we'd get an arrangement of "Fighting Fate." lol I also think his scale could be better realized in an action game like KH over FF13.

Thirdly, I'll say Kefka. Like Ultimecia, all the magic could be cool graphically and a cool (and annoying) gimmick to the fight. Assuming these boss fights get any dialogue or characterization outside of battle, having a chaotic "destroy everything CHAOS CHAOS CHAOS" villain would be refreshing compared to all the KH villains who want some sort of order or... some vague goal that doesn't involve destruction for insanity's sake. And with how long some of those KH boss fights take, they could get away with rearranging the entirety of "Dancing Mad." lol

Digressing some, I'm actually one of eight people who don't really care about FF cameos in KH. I mean, if they want to bring back any of the previously introduced characters, or use new ones to fill in for Story Relevant NPC's like in KH1 and 2, then I'm cool with that. But I personally did not miss them in KH3 at all; that game was troubled and bloated enough with all the plot points, characters, and Numbered KH mechanics it needed to include that forcing in unnecessary cameos would have just been awkward (unless they were like a random, non-story related summon or something). I would also rather see more of the original characters over watered down FF characters.

I concede superbosses are another thing and I wouldn't mind more of them in theory. My problem is that they'd only do it with characters I'm tired of seeing (Sephiroth) or have little interest in seeing (Lightning, Ardyn, anyone from 14 or 16 etc.). But that's a Me Problem and I won't begrudge them for doing it.

let's have a boss fight with Golbez and a Yoko remix of Battle with the Four Fiends

FFXIV Knights of the Round

Garland. I also wouldn't mind seeing Kingdom Hearts variations of the Final Fantasy villains, like Garland and Gilgamesh.