Dissidia Duodecim Final Fantasy possible characters

Kain, Lightning, and Tifa have been confirmed. Vaan's English voice actor has been confirmed for doing another FF-related project as Vaan, so most people assume Vaan has been slated to appear as well.

Most of the other FF games don't have a lot of options for new Dissidia characters. FF7 was kind of iffy because both Tifa and Vincent were so popular, but since Tifa is already confirmed, most people assume that there's not going to be another FF7 character.
FF6 and FF10 seem to have the most debate.

FF6 has any of Celes, Locke, Mog, or Edgar that make semi-logical choices for a new representative.
FF10 has Yuna, Auron, and Seymour as a distant third option.

Most talk seems to speculate that FF1 and FF3 probably aren't getting new reps.
FF11 and FF12 have the best probability to get more than one new rep.
If FF2 gets a rep, it's almost certainly going to be Leon.
If FF5 gets a rep, it's almost certainly going to be Gilgamesh.
If FF8 gets a rep, it's between Rinoa and Seifer (I'm leaning Seifer, personally).
If FF9 gets a rep, it's almost certainly going to be Vivi.

As a fan, I'd personally like to see Yuffie or Rikku in the game, but I doubt either of them would get in over Aerith/Yuna.