Hmm... based on my experience in the ORAS OU ladder, I'd probably say the Megas will end up something like this:
Ubers:
Salamence
OU:
Sableye, Slowbro, Altaria, Metagross, Lati@s, Gallade, Diancie
BL:
Beedrill
UU:
Camerupt, Lopunny, Sceptile, Swampert, Pidgeot, Sharpedo, Steelix, Glalie, Audino
I'm fairly positive the last five (Pidgeot, Sharpedo, Steelix, Glalie, Audino) will absolutely not make in in OU. I don't think Swampert will be OU just because rain teams in general aren't popular, and he's not good enough to change that on his own.
The highlighted Pokemon are all questionable. Sceptile and Lopunny are two great Pokemon, but they just don't hit quite hard enough by OU standards. I don't think either of them will be strong enough to be BL, though, as Sceptile is walled completely by Blissey and Florges (except for SD, which is walled by Mega Aggron). If Lopunny drops, it'll be a top tier threat as its ridic speed lets its beat offense and encore will help it break past defensive teams. Camerupt has a solid defensive typing and hits like a fucking nuke. It may be too strong for UU. Beedrill obviously hits like a truck and has drill run to plow through poison and steel types which resist its STABS. I really think u-turn sets are going to be too much for UU, particularly given how much easier it is to get rid of hazards in UU when compared to OU.
I'm really excited to try out Glalie in UU personally.