To be honest, Sephiroth just worked the best for the sort of gameplay that Kingdom Hearts utilizes. As much as I'd love to see Kuja or Kefka, they both rely heavily on magic, and it'd make the fight awkward if the enemy just kept running away and casting spells from a distance. Defininately difficult, but awkward.

Now a match against Seifer, Raijin and Fujin is a brilliant idea.