To be honest, the mega evolution possibilities are quite large in terms candidates in the Hoenn dex. we can definitely say that at least 7 pokemon are getting mega evolutions,(2 unofficially confirmed, 4 officials, and 1 from kalos), and that atleast one other if it holds any regard to the new character(flygon). That comes to a possible total of 8 so far, and with many other possible mon's as well(especially since we did receive a total of 28 megas in XY we could receive that same number if not lowered to below 20). My guess would be:
-Salamence- (I can feel it considering mence still has a large amount of popularity)
-Relicanth- (slim considering it's dex entry, but maybe mega evolution can change to what it was before or force a change. In that same respect, it would be the Aerodactyl treatment).
-Walrein- (feels odd thinking about it, but I could see gamefreak doing this to give another mega evolution to a ice type).
-Glalie- (Looking at glalie actually makes it seem like it could go further, and the likely hood of it might be possible. Just like Gardy receiving a mega, and still having gallade absent of one, Glalie can have a chance of receiving one despite having the froslass evolution).
-Milotic- (Milotic is a damn good candidate for a mega. Most would agree, just like mence. love it, or hate it I can see it being possible).
-Armaldo- (Honestly, it would start a new trend with evolving fossils, but it wouldn't be a bad start. Not super likely, but if they had to give another fossil mega evolution, Armaldo and cradily would be likely. Granted, that would throw relicanth out the window probably).
-Cradily- (See Armaldo. To be honest, one or the other. Would be nuts if both happened).
-Seviper- (I wouldn't doubt this one. The possibility of it even becoming Dragon wouldn't even feel far fetched to say the least. It or Zangoose, but it would be better in terms of there entries if both got one).
-Zangoose- (See Seviper)
-Altaria- (Another one I can possibly see, if not the first real Dragon/Fairy if they decide to do such a thing.)
-Flygon- (I already feel as if it's coming. Would be surprised if it didn't, but at the same time wouldn't. Conflicting views there).
-Torkoal- (would be nice).
-Sharpedo- (I feel as if it could be possible. Hoping for sharknado :P).
-wailord- (I'm not confident in this one, but hey, it could happen).
-Ludicolo- (Ludi is another one that makes sense. I can see it possibly happening).
-Shiftry- (Just like ludi, it makes sense).
-Lati twins- ( Unofficially confirmed. Just a matter of when it'll be official).
With all of that, I feel like a majority of those are strong possibilities(outside of the twins of course). If I had to guess all together it would be a total of 20 which includes the already known megas. this is also assuming it'll be all hoen mon's. Granted, they can easily go and add another 8 from a different region if they wanted to match the XY mega's.