Final Fantasy I:
- I'd actually leave this one with just the Warrior of Light and Garland. The game didn't actually have any characters and hell, the WoL himself was created specifically for Dissidia since the original game had no protagonist.
Final Fantasy II: LEON
- Leon was kind of an important character in FF2 and the fact that he actually had character development in the game would make him a good entry in Dissidia. I'd align him with Chaos originally but then he would move into Cosmos' army.
Final Fantasy III: XANDE
- Xande is the real antagonist in FF3, I can't explain why the Cloud of Darkness is there but anyway. Xande would make a great addition to Chaos' army since he didn't get any new fighters in Duodecim.
Final Fantasy IV: RYDIA
- Rydia would definitely be the best addition from FFIV. Just like Cecil and Kain, she would be aligned with Cosmos and gameplay-wise, I'd create some mix between Terra's spells and Yuna's summons.
Final Fantasy V: FARIS
- Faris would probably be a good character for the game, her gameplay style would allow her to change jobs on the go (Like Lightning's Paradigms) with something like Warrior, Black Mage and Ranger to allow her to do melee, ranged physical attacks and spells.
Final Fantasy VI: LOCKE
- Besides Terra, Locke is one of the most important heroes in FF6. Celes is great as well but that would leave us with another girl casting spells. Locke would be somewhat of a mix between the Onion Knight, Zidane and Tidus. He'd be using daggers to fight with great speed and evasiveness but he wouldn't be good from a distance since he's a thief.
Final Fantasy VII: VINCENT OR RENO
- I actually have two ideas for a new character representing FF7. Vincent has his own game and would be a great addition to Cosmos' side, in gameplay he would mix gunshots with materia spells to attack from a distance and in EX Mode he would be able to transform into any of the monsters he used in the games. On the other hand, Reno got insane popularity from his appearance in Advent Children. He'd be aligned with Chaos and the other villains would make a lot of fun of him. He could be a melee fighter with fast hits of his stick and kicks a-la Jecht and in EX-Mode He could call Rude to destroy his opponent with the helicopter from the movie.
Final Fantasy VIII: SEIFER
- For me, Seifer would be the best addition from FF8. He'd obviously be aligned with Chaos but to keep his style different from Squall. I'd give him more ranged attacks like spells and maybe the ability to call a GF or two.
Final Fantasy IX: VIVI
- Seriously, Vivi NEEDS to be in Dissidia. I'd make him like a magician version of the Onion Knight, with insanely fast spell combos from mid-range and slower ones for long-range. He'd be able to cast pretty much every element from the original FF9
Final Fantasy X: SEYMOUR
- I really can't explain why Square used Jecht in the first place when Seymour was the antagonist, just like in FF3 with Xande. Anyway, it's time for Seymour to get in Dissidia. He should be a mage with the ability to summon objects to throw them at his enemies, to keep him different from Exdeath, Golbez, Kuja and others.
Final Fantasy X-2: RIKKU
- Even though X-2 is not one of the mothership titles, Rikku is so freaking popular among the FF players (and female cosplayers who aren't fat) that she could fit into Dissidia's fanservice perfectly. She should be aligned with Cosmos obviously and probably just mess around with Bartz and Zidane in her story. Gameplay-wise I think she should be like a female Onion Knight, extremely fast and annoying but with a combo style like the one Prishe uses in Duodecim.
Final Fantasy XI: EALD'NARCHE
- He's one of the villains from the game's expansions and story-wise could fit into Dissidia; the problem I see with adding him is how to create a gameplay style that differs from Kuja, Golbez and other villains...
Final Fantasy XII: ASHE or BALTHIER
- Princess Ashe and Balthier are some of the most popular characters from FFXII. For me, Ashe should focus a little more on spells while Balthier could use attacks that involve guns and throwing grenades to keep them different from Vaan's style.
Final Fantasy XIII: DYSLEY and YAAG
- Both FF13 villains would obviously align with Chaos and while Dysley focuses on using the spells Barthendelus casted in the original FF13, Yaag could mix guns with summoning robots to his aid.
Other than that, I'd give one character from Tactics, Crystal Chronicles and 4 Heroes of Light because even though those are separate series, they are part of FF and should be added to Dissidia. In the case of FFXIV, I haven't played the game so I can't really say who should make it in.