I would have posted all of the characters alts ages ago...but they were ripped from the demo, which obviously we can't talk about. They are on the wiki now anyway, Yuna gets her wedding dress, so no gunner Yuna. I think her, Prishe, Gilgamesh and possibly Aeris are the only ones left to have their alternate outfits revealed.
You can select what costume you want your assists to use, that's why I mentioned Aeris. She'll probably get her Crisis Core and Before Crisis outfits if she gets any alts.