DISCLAIMER: I'm aware that Smogon tier doesn't accurately reflect how good these Megas are in game, but its a good way to get an idea of what we'll be seeing in E16.
FFA: Rayquaza (hahahahaha)
Uber: Blaziken, Mewtwo X, Mewtwo Y, Kangaskhan, Gengar, Lucario, Mawile, Salamence
OU: Alakazam, Altaria, Charizard X, Charizard Y, Diancie, Garchomp, Gardevoir, Gyarados, Lati@s, Lopunny, Manectric, Metagross, Sableye, Scizor, Slowbro, Tyranitar, Venusaur
BL: Gallade, Heracross, Medicham, Pinsir
UU: Abomasnow, Absol, Aerodactyl, Aggron, Ampharos, Beedrill, Blastoise, Pidgeot, Sceptile, Sharpedo, Swampert
BL2: Houndoom RU: Banette, Camerupt, Glalie, Steelix NU: Audino PU: Nium
Do with that what you will. It's safe to assume we won't be getting any of the legendary mega stones, and for E16, pokemon we haven't even gotten yet probably won't get one either. It's also a good bet that we won't get starter mega stones but that's always a possibility.