Anywhere between no chance at all and absolutely definite. The way they're going with the roster strikes an odd balance between clearly relevant (oras starters, megas, pikachu-libre) but at the same time they don't exactly feel there to advertise new features as such. At least, that's the way I'm feeling.
Would it help if I say it doesn't necessarily need to be a fighter, like it can be a multistage boss where you fight 10%, 50%, and then 100%.