Gamefreak has called upon YOU, yes you, to come up with the next series of Mega Pokemon. Probably a bad idea, but it's too late to turn back now. Here's the catch, they will only accept additional megas for Pokemon that already have them.

Do your best to make the new mega distinguish itself from the existing one, but also try to make it not overshadow it (except in the case of utter shit megas)
And remember to have fun!