I don't know how I feel, honestly. More of worrying about it screwing up the concepts of both of them for me.
Anyways, it seems that the Mega formes will be triggered by an item [e.g. Lucarionite, Blazikenite, etc.]. However, it is still vague [to me] if it will be a Stone item [consumable, one-time use, one-way (e.g. Thunderstone, Moon Stone, etc.)] or it will be a Hold item [See: Arceus- Multitype and Plates].