Well X and Y got 26 mega evolutions. The ones we do know about that are in this game are-
Mega- Swampert, Sceptile, Blaizeken, Latias, Latios, Metagross, Groudon, Kyogre, Mawile, Sableye, Absol, Aggron, Banette, Pinsir, Scizor, Gyarados and Gardevoir
As you can see its a ♥♥♥♥ ton of megas. So, its safe to assume we won't be getting the same amount as X and Y
My prediction for some of the more unclear megas are
"Mega" Kyogre will be water/electric to take advantage of its weather and to give groudon some leverage
Mega metagross will get tough claws. Its ind of obvious because gamefreak actually gave the mega claws whereas regular metagross has none
My prediction for new megas are-
Camerupt, Sharpedo, Possibly Crawdaunt, Shiftry and Cacturne
I can also see Glailie, Walrein or Wailord getting one
The last ones are a bit of a stretch but-
Linoone because its probably the most buffed first road derp in the games
Castform because its a hoenn special
and mightyena the other most buffed first road catch