Not really a fan of the cosplay thing, maybe because its only focused on Pikachu (so far as we know),and Pikachu annoys the hell out of me.
the Contests are interesting. I would love it if they made it like the anime and had you battle the other trainers to show off your moves. I like the idea that you can Mega Evolve during the contest for more appeal, but, will the other trainers be able to do this too? That would make it more engaging and fun.
As for mega pokemon, I love the look of Mega Lopunny and Mega Slowbro. Mega Audino...meh. Sure it will be good against the Dragons, but other things....not too sure.
I'm hoping for a Mega Pidgeot, Mega Arbok, Mega Rapidash, Mega Mightyena or a Mega Flygon.
Outfit changes, I'm hoping make a return, though probably won't. I love the idea of changing the players clothes, helps make the trainer your own.