1) Male, All-New to Dissidia, Later-Era FF games.
2) Male, All-New to Dissidia, Early-Era FF games. (Could be Minwu or Locke)
3) Female, All-New to Dissidia, Later-Era FF games.
4) Male, All-New to Dissidia, Later-Era FF games.
5) Either Yuna, Prishe, or Tifa.
6) Male, All-New to Dissidia, Later-Era FF games.
Alright, so the first thing to note is that there are five characters from the later games. There are also five numbered games that have only one representative, which implies that they are going to emphasis balance. This tends to favour Prishe for 5), although I wont say Yuna doesn't have a chance (also, Tifa is from the first half of FF, so she isn't an option)
Secondly, with the whole event late last year with Locke's VA, Locke is easily the most likely option for 2)
As for 3), there are several choices. In numerical order we could go with:
Lady Lilith (most popular villain from FFXI) or Lilisette (third most popular XI character after Shantotto and Prishe) from XI: Although this assumes Prishe doesn't get in
Ashe from XII - next most prominent character in XII
Serah from XIII - Lead character from XIII-2, also pretty popular in Japan
Eula Darnus - Mentioned by Yoshi P as one of his preferred characters
Aranea Highwind - Really popular female character
Of these Female characters, I think Serah has the best chances, simply because her two biggest competitors, Snow and Raines aren't as big as the competition most of the others have to face
As for the three remaining men
XII - Vayne or Baltheir
XIV - Zenos
XV - Ardyn.
A few notes. If Serah isn't the new female, I expect Raines to get in before her, to give XIII a villain. If Yuna is returning female, either XI or XV will miss out on a rep this year. Probably XI
4) (Vayne or Baltheir) Or Ardyn
6) (Vayne or Baltheir) or Ardyn