I'm thinking they will just reveal the new Megas, Brendan and May's (hopefully) updated artworks, and obviously Hoenn in 3D, like everyone wanted.
I'd like to know if we'll be able to customize our players clothing and hair. I honestly wouldn't mind if we can only wear Brendan and May's default uniform, but I wouldn't be opposed to customizing them either. Along with Mega evolutions and a look at Hoenn, those are the main things I hope are revealed.