Yes. Probably every fully-evolved Pokemon will get a Mega Evolution, I speculate (even Delibird).
My two issues, which have been broached at length, but I will reiterate for posterity, with Mega Pokemon:
1. It takes a turn to activate
2. You don't have an item
Both, in tandem, almost certainly compromise the strategic viability of a Mega Pokemon. The fact that you can only have one also mitigates their usability; the only ones I think are of particularly good use are MegaMewtwo and MegaLucario, the one because of absurdly high Special Attack, and the other because of (probably) Base 140 Attack + STAB/Adaptability Close Combat. The prospect of MegaMawile is indeed high, but again I'm worried about the use of a turn. Does Mega Evolving have priority? If not, then MegaMawile will be in serious trouble, one Fire Blast/STAB Earth Power/STAB EQ and he's down to red, so you have to play your cards right. His severe Attack is undermined by his lacking Speed, since he needs both to be a contender, I think (Sucker Punch would give him an edge, but not an impressive enough one).
The two mentioned above (MegaMewtwo, MegaLuke) are probably the best on paper, but I would also give honorable mention to MegaBlaziken, because it will probably have around Base 130 Attack, with Speed Boost it will be a veritable sweeper. Also MegaBlastoise, whose Hydro Pump may very well be on par with Haxorus's Outrage in damage output thanks to Mega Launcher, is wortyh of attention, but his new ability falls well within the field of gimmick, as Hydro Pump will be his only good "pulse" attack - other than.... Water Pulse? Flash Cannon? The latter will be better offensively because of the new type, but come on. I'm not terribly amused.
At this point, we don't really know enough to make any real speculation about what kind of impact these new Pokemon will have in the competitive sphere. I have a feeling that they may, in the end, be a wasted slot on any team, because their only purpose is to come in there and scare the s*** out of your opponent, but then end up falling from one or two attacks. They're almost like the Pokemon equivalent of Final Smashes from SSBB - some Pokemon get better ones than others, but no one will end up using them (for one reason or another). tl;dr: if a Mega Pokemon has low Speed, don't use it.