I'd rather not see Sephiroth again, as Sorarocks93 pointed out it'd be a bit repetitive.
However I'd love to see some more disney bosses like Julius or maybe some FF summons as bosses~
I personally think Sephiroth should be in KH3 just to end the Cloud-Sephiroth subplot. I mean, I understand it's repetitive, but it seems weird if they introduced this very small plot between the two in KH1FM (that everyone will get to see in 1.5HD) and continued it in KH2 and then just never bring it back up.
While I agree that it would be repetitive, think of this as the last time you'll face Sephy or see Cloud, I think that wouldn't be so bad.
I wouldn't debate that, at least for the diehard fans that actually knew about the Final Mix games to begin with. However, for the more casual fans that only knew of the American version of the first game, the Cloud vs Sephiroth subplot started with KH2.
The subplot will be in KH1FM of KH1.5 HD Remix so all fans will get to see that it started in KH1
As for bosses I'd love to see, I think I mentioned them in the other "superbosses" thread but I'd love to see:
Ultimecia
Kefka
All the Titans from Hercules