So Pokemon can now mega evolve into stronger forms, you can have one per team(I think), and they will be so strong most will be able to turn the tide of battle.

Enter ditto. ditto can copy both stat changes why not form changes? Worst case scenario he'll just revert back to the Pokemon's normal form because he doesn't have a stone. That'll mean all you need to stop mega forms is knock-off. If not holy shit choice scarf imposter ditto for new king of meta