At some point I think it would be cool to do one for Ultimecia's Castle from FFVIII someday. I mean, there's the Omega Weapon in the castle, if nothing else. It could be an actual revamped version of the Ultima fight with new mechanics that make it a pretty good challenge. I haven't tried Proto Ultima yet, but the consensus seems to be that it was kind of disappointing even when it was relevant. Either way, the castle had a ton of bosses to fight, and the time/space shenanigans make Ultimecia a decent candidate for a cameo too.