Mega Evolution is mainly a popularity contest right now. That's why Mewtwo and Charizard have two. And Pokemon like Blastoise, Venusaur, Garchomp, Lucario, Blaziken, Tyranitar, Aggron, Gengar, Alakazam, Gyarados and such didn't really need Mega Forms because they are already powerful and didn't need the boost, and now some of them are OP or more OP then they were before. Pokemon like Mawhile, Absol, Heracross, Pinsir, Manectric, Medicham and Houndoom on the other hand were mostly fine getting new forms and boost their power. Pokemon that could have really benefited from Mega Evolutions didn't get the Mega Forms and instead the popular Pokemon got them. I'll always wonder why Blaziken got a Mega but the other two Hoenn Starters didn't. Heck, the Johto, Sinnoh, and Unova ones didn't get any Megas, nor did Kalos. But I can see Greninja getting one. He's being shoved down everyone's throats. For the most part Mega Evolution is a popularity contest and several Pokemon that could have used a new form didn't just to give the populars more exposure. For me that is a missed opportunity.
Parena or Serecham both equal Serena+Pancham! Sereevee or Serevee equals Serena+Eevee!