Charizard has two megas which Venu / Blastoise didn't receive, even with possible follow up games to expand on them (ORAS). Greninja is highly popular so preferential treatment that also has precedent isn't really out of the blue lol.
In terms on how you'd establish this in a follow-up game, appearance-wise could just be customization or something AZ-esque as "the transformation occurred in Kalos 1000 years ago". Activation could be something ala Mega Rayquaza and require Water Shuriken as a move to activate the m-evo.
In terms on how you'd establish this in a follow-up game, appearance-wise could just be customization or something AZ-esque as "the transformation occurred in Kalos 1000 years ago". Activation could be something ala Mega Rayquaza and require Water Shuriken as a move to activate the m-evo.