If Celes were added she may be a Cosmos character. It seems pretty evident from the trailer that we're exploring Terra's time as a slave to Kefka. If the rumors of Sephy controlling Cloud are true and with Kain's usual jealous/asshole shenanigans, we may be looking at a lot of backstabbing going on.

Tidus throws me off a bit. Unlike Terra, Cloud or Kain, he doesn't have anything in his story that would make me feel he would be on Chaos' side. I'm still betting his neutral at best. It's not like the Cosmos side didn't fight each other at times. I'm also curious about Kuja as well since the trailers are hinting he's on cosmos side or at least mistaken for their kind. Like Tidus, he has nothing from his own game that makes me feel like he would be on cosmos side naturally. Even if they added IX's version of Garland as a bad guy, its not like Kuja's goals were actual noble, understandable, but hardly noble.