Okay, now that the method of mega-evolution is pretty clear now, I think we can assess which mega-evos will have an impact, and which won't.
1. MegaBlaziken seems pretty useless, since you'd have to waste one turn to Mega-evolve, and it doesn't seem like Blaziken actuslly gets any benefits from it's evolution.
2. MegaLucario is in the same boat too, you could just use swords dance rather than use a turn and waste an item Megaevolving.
3. MegaMawile actually seems like it would see some use. Intimidate would help you mega-evolve easily against physical attackers, and then you'd be able to hit stuff with huge-power boosted atracks.
You don't have to waste a turn to mega-evolve. You can move in the same turn as you do so.
Because of this, needless to say Megablaziken will stay in Ubers. Even better Speed Boost Blaziken? oh dear.
Provided Lucario gets a small speed boost (let's say to 105) then I could see it Ubers honestly. 4MSS doesn't matter much when you can fire off +2 240 BP Close Combats and 80 BP Bullet Punches. Of course to make a true judgement we'd need its base stats first.