Surprise, I bet there are even more still
CoroCoro magazine has revealed a gaggle of additional Mega Evolutions to appear in Pokémon Omega Ruby and Pokémon Alpha Sapphire, including Mega Altaria, Mega Lopunny, and Mega Salamence. It seems that these are a bit of a random assortment of monsters being chosen for Mega Evolutions, or maybe there's a method to the madness. Who can say for sure?
Also available will be Shiny Beldum, which comes already holding the stone it needs for a Mega Evolution into a Mega Metagross.
There's also some information regarding contests that came out of this issue of CoroCoro, specifically the fact that an NPC named Lucia will offer you special outfits for participating in Pokémon contests.
Oh, Omega Ruby and Alpha Sapphire. Why aren't you here yet?