The subject of this thread is simple: Make a Mega, and try not to make it ridiculously broken. I mean, you can if you want. I'm not your dad. I can't be all like "That's irresponsible, why would you do that, I have no son" and force you into hiding yourself in your room and causing deep psychological trauma.

My point is, go crazy, but try to be realistic.

That being said, I'll start.
Mega Archeops
Rock/Flying
Ability: Underdog
>Halves this Pokemon's Attack and Special Attack when health is yellow (50%), but doubles Attack and Special Attack when health is red (25%).
HP: 75 -> 75
Attack: 140 -> 180 (+40)
Defense: 65 -> 75 (+10)
Sp. Attack: 112 -> 122 (+10)
Sp. Defense: 65 -> 75 (+10)
Speed: 110 -> 140 (+30)