As of now, the games have tried to be a bit more reserved with creating Mega Evolution forms. There are 48 possible Mega Evolutions, which is only around five percent of all Pokémon. That said ...
Not all Pokemon capable of Mega Evolution may return in Z-A. Mega Kangaskhan, Mega Pinsir, and Mega Houndoom are highlighted as powerful additions. As it's certainly probable that new Mega forms ...