Sephiroth because he's only been a big part of the series in 2 games. This subplot is not finished, so why not finish it now? Please, i can only imagine how crazy his boss fight would be on a next gen console. Imagine there's a portal and you're wisked to midgar or something. What if you fight him in that place where cloud fought him in Advent Children? Oh boi
Exactly, Sephiroth has become almost a staple in the series by now and his subplot is left hanging, but they DO have to spice him up and make him actually dangerous so that not just leveling up to 99 and learning his moves is already enough to sure-fire defeat him.
He would also definitely need new moves and some surprises up his sleeve (like i.e. his Scintilla-technique from Dissidia where he blocks an attack, stuns his opponent and counters with a heavy two-part sword slash).
Maybe even a two-phase battle against him would be neat where you only fight him in human form during phase I while in phase II he'll switch to One-Winged-Angel form and that's also when you get Cloud (and maybe Zack too?) as party member to be able to fight him on even grounds.
It would also be a sort of sneak peak for the FF VII remake which SE will surely not let pass.
However, that doesn't have to (and shouldn't) mean that he's the only secret non-Disney boss, Vanilla KH2 was very boring in that regard and you couldn't even re-challenge him like in KH 1.
As for other Final Fantasy bosses, I'm all in for those, especially since Dissidia already gave us some glimpses at how they fight.
Having to use the new freeflow movements in order to keep up with a fast, floating Kuja who's always shooting Flare and Holy magic at you from all angles or trying to dodge Golbez' Genesis rock could surely be interesting.
