Thanks to PsychicNature for suggesting this mega evolution!
Alright. So Mega Mew gets the protean ability, changing it into whatever move type it uses. Because mew can use so many moves, this takes it to a whole new level of diversity and randomness.
Mega mew gets a boost to all of its stats. Its appearance doesent change too much, but it does get a mega evolution symbol on its arms and three tails, as well as spikier ears and a deeper shade of pink.
Stat boosts:
20attack
20 defense
20 sp attack
20 sp defense
20 speed