Copy/paste from Dissidia forums:
Here are the current changes that are rumored, seen or known for the cast that has appeared in previous games
Warrior of Light:
Shield of Light is now a Brave Move (No move name is seen when he uses it in Y'shtola's Trailer)
Onion Knight:
Can now freely change between Sage and Ninja in battle 'is now an EX Skill'
Blizzard now Lingers, OK can freely move once casted, knocks player upwards if hit
Terra:
Trance is now her main EX Skill
Meteor is now a HP attack
Possible Running Speed buff
website hints that she can freely move when casting any spell
Cloud:
Finishing Touch now produces a tornado and is now a brave attack
NEW MOVE: Spin Drive, can be used while dashing 'part of new dash move slot'
Cloud is said to have a system where holding O charges his brave attacks and makes his attacks stronger
Cross Slash and Meteor rain are returning, unknown at this stage of attack type
Lightning:
She no longer can change to Medic stance
Bartz:
Play Style will be changing according to his VA, will be able to change jobs and have 1 ability per job, loses copycat play style.
Green = Confirmed
Orange = Rumored or Not enough information known
These are my predictions based on popularity, inclusion in other media like Kingdom Hearts and Theatrhythm, and story line importance. Wonder how many are right
FFIV - Rydia or Edge
FFVI - Locke, Celes, Setzer or Edgar
FFVII - Aerith (fully playable this time), Vincent, Yuffie or Red XIII.
FFVIII - Rinoa and Seifer
FFIX - Vivi, Garnet and/or Beatrix
FFX - Auron and Seymour
FFXII - Fran, Balthier or Ashe
FFXIII - Hope, Snow, Vanille or Fang
FFXV - Noctis
FFT0 - Ace, Rem or Machina
I'll explain why I chose these:
Rydia and Edge are the main party members in FFIV after Cecil and Kain, and in the originals you had no choice but to use them. Both of them were also chosen for Theatrhythm Curtain Call.
Locke, Celes, Setzer and Edgar are also the only party members you have to take to Kefka's tower to finish FFVI. Behind Terra, these guys are the most important, not to mention Locke was also considered for the original Dissidia. Locke, Celes and Edgar are also in Theatrhythm Curtain Call, and Setzer in Kingdom Hearts.
Aerith is the 3rd most important party member behind Cloud and Tifa, as she also has beef with Sephiroth. Like the other characters in VII she is incredibly popular, and she was already included in Duodecim. Yuffie is also incredibly popular, she doesn't have any connection story wise, and she's not connected to any of the other characters really, she has appeared in every entry in the compilation with a lot of screen time. She is also a boss in Kingdom Hearts I and II, so there's a moveset they can expand on. Vincent has a lot of ties, since he tried to stop Sephiroth's birth and feels a guilt for his existence, and what it meant for Lucrecia. He has appeared in most compilation titles, even with his own game. He is unique with his transformations into monsters, and a fan favorite. Red XIII isn't too important within the compilation, often being left out, but I predicted him because of his uniqueness of being a Dog. We need more non human/humanoid characters. All of these characters besides Red XIII were included in Theatrhythm Curtain Call.
FFVIII's plot revolves around Squall and Rinoa's romance. She's even in the game's logo. Behind Laguna, she makes the most sense for the next character. Seifer is also the most logical choice for VIII, since he was left out of the original, and he's probably the only other significant antagonist in the game. Seifer was included in Kingdom Hearts II, and both of them were included in Theatrhythm Curtain Call.
Vivi and Garnet both have strong ties to Kuja. Vivi was created by him, Garnet's mother was manipulated by him, and both of them are essential to the plot of FFIX. Vivi was included in Kingdom Hearts II, and both of them were included in Theatrhythm Curtain Call. Beatrix is incredibly popular, and I chose her because of her importance in the story of IX, with her appearing from time to time and destroying the party.
Auron and Seymour are both important to the plot of FFX. The games events wouldn't have happened without Auron, and Seymour is weirdly obsessed with Sin and Yuna. Not to mention Seymour was dropped from the original Dissidia. Auron was also included in Kingdom Hearts II and Theatrhythm Curtain Call.
I haven't finished XII, but these 3 are very popular in the fan base. Balthier was dropped from the original Dissidia as well. All 3 were included in Theatrhythm Curtain Call.
Hope and Vanille are most likely for XIII, since they got the most development out of the cast. Hope is a big character in the XIII series, always in a major role trying to help everyone. Vanille set the whole game into motion by hiding her focus, and a theme in the series is her guilt for not helping Fang when she had the chance. Vanille was originally the main character of XIII, being replaced by Lightning. Fang is also important to the story, with her relationship with Vanille and the war of transgression. She is also really popular. I chose Snow because he has close ties to Lightning, and he seems to be popular. All of these apart from Fang were included in Theatrhythm Curtain Call.
Noctis makes sense because he is the only playable character, but he might not be included right away if FFXV is not released before this, for the same reason Lightning was dropped from the original.
Ace, Machina and Rem are the most important characters in Type 0, and they are the most advertised. All of them are included in Theatrhythm Curtain Call.