Oddly enough... Kefka is actually one of the few FF villains in Dissidia whose entire move set is actually based on abilities he actually has in his original game (beyond the goofy names and effects of course). Most are taken directly out of his move set from his final boss form making him the only villain who doesn't actually have completely made up moves for the game or ones that are borrowed from other titles.

Back on topic...

My ideas:

Kefka - Flee, 100% escape rate from battles.

Gestahl - Command, summons low ranking Imperial troops to act as shields for him. Occasionally summons Leo who does shock on enemies and disappears and occasionally summons Kefka who flees on the spot.

Siegfried - Hyperdrive, cause its his bull super move in the arena...

Ultros - Tentacle, acts like the counter moves, allows Ultros to counterattack the whole party when magic is used on him

Chupon - Sneeze, obvious...

Impressario - coverup, allows him to reset the last action in battle. If a character is killed, Impressario creates an elaborate lie that makes it so they live, same with enemy damage...

Call Girl - Humpty and Dumpty, makes everyone lose a turn.