I still say its too early to say if the ? is just Vaan.

Honestly, I still feel that Leon has a good chance. It's IX I feel may have issues garnering another worthwhile character out of the game. Only Vivi and Dagger have actual relationships with both Zidane and Kuja. I feel both would create issues trying to make builds for their fighting styles and have issues fitting into the plot. Still, I'm really hoping for Vivi cause he has the better chance and I like him more.

I guess my main concern is that Lightning still needs a villain (though I wouldn't mind if they decided to opt out cause XIII never had good villains to begin with) and I just can't get over this nagging feeling that SE will cave into the Japanese fandom and bring both Yuna and Seymour into the game, since both were originally considered. That's why I feel a few FFs might get the flack so they bring two characters from a numbered FF instead of one. Hell, I'm still waiting for Zack Fair to be announced to represent Crisis Core and be a secret character.

As for Xande/CoD vs. Golbez/Zemus; there is one major factor you are forgetting here. The OKs don't have an actual relationship with either one of III's villains. Xande may have had more to do with the plot but he still technically a random evil bad guy to the main characters. They don't know each other and they don't even meet til the very end of the game. Golbez was chosen cause he actually has a history with Cecil. Considering Kain was originally considered before Golbez by the development team should show that Golbez wasn't special to begin with.

CoD I felt was chosen cause it represents the true main theme and conflict of III, a battle for balance and the need for light and darkness. If it was Xande, it would simply be about him. Since Dissidia pretty much takes most of the characters out of their environments and scenarios; Xande would completely lose what made him an interesting villain to begin with. He would just be another "I R EVIL" wizard. It's like how some of the villains like Kuja, Sephiroth and ExDeath for example, are kinda flat in Dissidia. Because once you remove their main conflicts you realize they don't have much else going for them.