The stadiums shown in the debut trailer will relate to some new gimmick - and part of it will involve online functionality.
I also think we're going to get some large, open field areas with visible Pokemon (random encounters reserved for linear routes and dungeons). The region map is an odd shape, and it looks like a couple of spaces could be freely explorable (by Pokemon standards). I don't think this'll change the progression of the game much, because Game Freak will probably limit where you can go through plot devices and special moves, but I'd like it if we had a couple of open areas to explore. One thing they could do is change which Pokemon inhabit the open areas depending on certain conditions - day/night cycle and seasonal differences (or just weather changes) would be a way of doing it. It'd also tie in to special events, with event Pokemon being available in the field areas over a weekend or something.