Sad to say, a Dissidia 2 would probably suck.
----What Dissidia 2 SHOULD Be----
The original cast of heroes should return, but the villains will change for some. Dissidia 2's villain line-up will be thus - Garland, Leon, Xande, Kain, Gilgamesh, Celes, Sephiroth, Seifer, Kuja. For X, we may see Yuna and Seymour. Shantotto and Gabranth will be opposed by the Shadow Lord, and....well for XII you could use Vaan, Ashe or Basch, depending on what type of story and fighting style you want. Depending on the timeframe we'll also get XIII characters.
----What Dissidia 2 MAY Be (is likely to be)----
All the original cast return, all of them. New characters are chosen by fan request, so with a few exceptions all the new characters will be from the Sony Era, and more than half the cast will be from FF7, 8, 10 and 12. Because there's so many characters now, the plot is not as simplistic as the first time and turns off fans of the original. The fighting system will introduce a few new concepts but ultimately it will play the same as the first.
OR, and this makes sense but is still a bad idea, to fit the concept of Dissidia's "cycle of war", we get twenty all new characters from the various spin-offs, but the spin-offs don't have nearly the same following. So with a few exceptions, most of the new characters will be from the Ivalice or FF7 universes, and thus the game will have a much smaller following because its characters are all Ivalice and FF7-based.
