*coughfirstmegaevolutionannouncedwasMewtwocough*
I think that they're eventually going to make mega everything. Keep in mind that a lot of the more Powerful Pokemon lose some stat or another upon ME. For example, Mega Garchomp's defense is crap, and Mega Aggron loses a type.
Garchomp: 108/95/85
Mega-Garchomp: 108/115/95
Seems legit...
MChomp sucks because of the speed decrease, not the defensive *increase*, and Aggron losing it's hideous rock typing is great.
I still say that Mega Evolutions are more of a power shift than a power up in most cases.
Gardevoir, Heracross, both Charizards (though more X), Venusaur, Gengar, Kangaskhan, Pinsir, Gyarados, Aerodactyl, Scizor, Tyranitar, lol Blaziken, Mawile, Medicham, Manectric, and Lucario would say otherwise.