Just a theory here, did Nintendo hint to us which additional Pokemon will get a Mega Evolution? If we look at the trailers and demos, all of the Pokemon on the receiving end of the attacks are their final evolutions.

List of Pokemon on receiving end:
Gyarados
Hydreigon
Tyranitar
Golurk
Salamence
Genesect
Rampardos
Zangoose
Cloyster
Machamp
Zangoose
Crobat
Chandelure
Dragonite