Mega Stones no longer are needed to mega evolve. Instead, like Rayquaza, every pokemon needs to have a specific move, let's say in it's first moveslot (for no double mega shenanigans), in order to mega evolve, exclusive for each pokemon.
Which move does your mega needs to mega and why?
How much better or worse is it now? (optional)

I'll start
>Medicham needs to know Meditate to mega evolve
>Mewtwo needs Bulk Up for X, Calm Mind for Y
>Gyarados needs to know Crunch
>Charizard needs Outrage for X and Overheat for Y