I’m actually kind of surprised we haven’t already seen a Mega Pikachu. I’m not in the habit of making predictions, but there’s a tentative one for you; Pikachu will eventually Mega Evolve. Having said that, this is sort of a tricky question – not for gameplay reasons because, like, sure, whatever (most unevolved Pokémon would need much larger stat increases than normal in order to be worth using over another Mega Pokémon, but there’s no reason we can’t give that to them); more so for whether and how it fits into the world. At the Tower of Mastery in Shalour City, the Guru calls Mega Evolution “a transformation of Pokémon that were thought to be unable to evolve any further,” and while it’s pretty clear that the characters in X and Y do not by any means know all the rules of Mega Evolution, not even him, I think it’s telling that the writers would choose to emphasise the fact that only fully-evolved Pokémon do it – he actually mentions this before bringing up the fact that it’s temporary, which seems like it should be a more important point. I think there may be an idea in play that, because Mega Evolution “transcends” normal evolution, because it’s above and beyond normal evolution, it would not make sense for Pokémon to be able to do it before completing their normal evolution. Unless the evolved Pokémon can also Mega Evolve (in which case why are you even using the younger one?) then evolution for that species would become a loss, a sacrifice; it would mean being cut off from this “transcendant” state. I don’t think Game Freak would like that notion.