Seifer was the secondary villain of VIII, much like Seymour was the secondary villain of X, the Turks were the secondary villains of VII, Beatrix was the secondary villain of IX (debatable, but she comes the closest, even though she's also a good guy), and, yes, Gilgamesh was the secondary villain of V.
These characters show up in pretty much every FF game, they have quite a bit of plot behind them, and would certainly be my first choice for new villains for a second Dissidia game (except Seymour, 'cause, you know, he's a terrible character who should never have existed).
And you lose points for failing to see just how awesome having Gilgamesh in this game would be.![]()