Ok /vp/ here's my idea. GameFreak should make a new move, learned only by a special pokemon (maybe a legendary, a weak one) that de-evolves pokemons.
The thing is, now the code to change a pokemon's sprite, type and ability during battle is already in-game (mega evolutions), so it wouldn't be that difficult. A basic pokemon would take no effect, a mega-pokemon would revert back to his normal form (with no chance of megaevolving again: note that if you revive a poke that megaevolved before and fainted, you cannot megaevolve it again), and an evolved pokemon would deevolve. Obviously you can use this move only once on each pokemon.
I don't know if this move should last all battle or maybe five turns, and I was looking forward to hear your opinion.
Note that the pokemon learning it should be really weak, like this is his only ability and he's shit. I think a pokemon like Hoopa could be the concept to build this on.
What do you think?