Oh my god. These mega-evolutions make me a little scared, because I don't want to see Pokémon becoming Digimon-like. I really hope these evolutions will not be actual ones and these Mega forms will be referenced by the Pokédex as the same species as the original ones, because I'm sorry but overdesign a Pokémon and adding "Mega" in its name isn't enough to consifer it as an actual evolution. I also hope these mega evolution will always have the same base stat total as their regular forms, but reorganized. I wonder if gen VI Pokémon and not only older gens Pokémon will get Mega forms. I also hope Pokémon with mega evolutions/forms will not be able to get Eviolite boost, actually I think this method of "evolution" was included in these games to avoid Eviolite issues (Mawile not getting Eviolite boost in gen V but getting it is Gen VI).
About designs, the only Mega form I quite like is Blaziken's, it looks fine IMO, but I don't like the other Mega design, theye are just uglier, overdesigned versions of regular Pokémon (the worst being Ampharos). Every gen that included a new Eeveelution also included a new member to Electabuzz, Magmar and Porygon families, so I expect a mega form for Electivire, Magmortar and Porygon-Z, but I hope some of my favourite Pokémon like Skarmory and Terrakion will not get terrible-looking mega forms.