In the next generation, pseudo mega stones that are consumed when used to mega evolve a Pokemon exist. Mega evolution done with them has a limit of 10 turns, during which the pokemon becomes one rank higher in Attack, Defense, Special Attack, Special Defense, and Speed per turn.
When past the 10 turn limit, the Pokemon faints and its mega evolution is undone. Withdrawing the Pokemon prevents the turn count from going down.
Adding to this, the normal mega stones never get released. How does this change the meta?