So who would you change out if you got to choose? like Seymour for Jecht, Seifer for ultimecia, ect.... i will make a poll once you throw out some choices.

Cloud Strife for Zack Fair
Sephiroth for Genesis
Tidus for Yuna
Just to add Lightning from FFXIII
Jecht for Auron
Gabranth for Basch or Ashe
Add some FFX-2 Characters:
Paine
Rikku
Shuyin
And some add on music would be nice not that the soundtrack sucks or anything.

since I see that the topic has turned to "what characters would you replace, and with whom?", I can offer my two cents...

There are only a few characters I would replace...First off, I would certainly replace the "Light Warrior" with a character that looks more like one of the original sprites from Final Fantasy I...the "Light Warrior" in Dissidia has only been around since the PS1 Origins set...hardly representative of the original FF1 game...next, I would choose to replace Terra from FFVI (I know, a controversial choice, most likely) with Edgar, Cyan, Locke or Sabin....take your pick...

Cloud and Sephiroth are good for VII, VIII - I would change Ultimecia for Seifer, and maybe add Laguna as an extra character...X, I might just add Seymour as an extra character because Jecht is a great foil for Tidus...the one I would most like to change, however, is the character they chose from FFXI online...I would make it either one of the Cat-girls (forget what they are called) or one of the giant Beast-Men (again, forget what SE named them)...either one would make a great addition to the game, and fit in better than the midget magician-hobbit-thing...