With the sequel to Dissidia recently being announced, there's been a lot of discussion about the roster and who should or shouldn't be included and it seems my opinion differs with what most people are expecting, so I thought it would be worth putting together a list of the characters that I'd personally like to see in Dissidia 2.  And yes, I'm going to continue calling it that, as the official name is rather stupid.
 
First of all, I'd like to say that I was disappointed with the lack of female characters included in the first game.  Obviously, since they were going with the primary villian and main characters from each game, this was always going to be the case but the one female lead character who did make it in, Terra, was implemented very poorly.  While the rest of the characters stayed relatively true to how they acted in their original games, Terra was reduced to the typical submissive JRPG female in distress.  She was weak, timid and constantly in need of protection from a 12 year old boy.  Where as in VI, she may have been confused and sometimes unsure of herself but she was ultimately brave and assertive.  Her portrayal in Dissidia was not only bad but just plain sexist.
 
So with that being said, the majority of my picks are going to be the females, not just because they were poorly portrayed in Dissidia but also because I feel they would be the best fits.
 

Final Fantasy IV

Since he's already been announced, there's little need for discussion here.  Kain was one of my favourite characters from any of the games, he was complex, interesting and very cool.  He was the obvious pick from IV and should make for a very unique character in Dissidia with Dragoon abilities.  His ex-burst is also his Holy Dragoon form from The After Years, which is a pretty nice reference to the FFIV sequel, even if the game was pretty dire.

Final Fantasy V
I think FFV is one most people think they'll skip over and it honestly wouldn't surprise me either.  I'll admit that I was never a big fan of the game but I did really like the character of Faris.  Pirates are always cool and she was one of the only aspects of the game that really stood out to me.  I think her piratey style would make a great contrast against most of the knightly characters.

Final Fantasy VI

Refering back to my desire for more strong female characters in the game, Celes fits this perfectly.  She's actually considered the main character of the game by many, replacing Terra for the second half of the game, having more key sequences and having a strong connection to the main villian of the game.  Many would prefer Locke but I think his attitude and fighting style would just be too similar to Zidane's.  Celes is just the better fit.

Final Fantasy VII
What needs to be said here?  I'm sure everybody already knows her and she's already confirmed to be in the game.  I will say that I'm glad they went with her over Vincent, who I strongly suspected would have been included to appease the fanboys.  Thankfully, they went with the more significant character and one that should have a very unique style in the game.

Final Fantasy VIII

The man with the machine gun!  This is another where I think my opinion differes from most.  Bets seem to be on either Seifer or Rinoa but I think Laguna is the more logical choice.  He was a great contrast to Squall's character in VIII and one of the more interesting from any of the FF's, I would have loved to see a whole game centered around him.  Using a gun would again make him rather unique against the majority of heroes who all use swords and I think he'd an interesting diemension to the story elements.   While I have said I'd prefer to see more female leads, VIII doesn't have much in the way of good characters, particularly female ones.  Quistis did very little, Selphie was the standard "Yuffie" character and Rinoa was all round unlikable and a poor fighter.  Seifer would make a good rival for Squall, but I think he'd be too similar of a character and again, I think the elements Laguna could add would be more interesting.

Final Fantasy IX

This one I'll admit is rather unlikely, call it something of a fan wish.  She wasn't a main party member and wasn't really anything of a villian or even a rival.  But she was a really great character, had fantastic battle music and was just generally awesome.  Again, I'd call her one of the most stand out of the entire series.     I think Vivi is the more logical and likely choice here and would add a good magic user for the heroes..  But God damn, it should be her.

Oh! I forgot all about Beatrix :) It's been so long since I played IX, even though I liked that game more than many, it seems.
 
Looks like so far they haven't announced any IX characters at all, which is pretty surprising to me.