This was unexpected.
Mega Slowbro gets Shell Armor, which, unless the ability gets an upgrade, makes me write off this Mega immediately. Regenerator is a lot more useful.
Mega Audino becomes a Normal/Fairy that gets the ability Healer. Which, again, kind of sucks. 30% Chance to heal an ally isn't that useful in Doubles. Here's hoping the stat-boosts make Audino more viable though.
That said, my theory is that our remaining Megas are; Primal Rayquaza Alpha, Primal Rayquaza Omega, Mega Glalie/Walrein, Mega Milotic, Mega Sharpedo and Mega Camerupt.
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Also,
I have another theory that I really hope is true. All the Elite Four have Potential Mega Hoenn Pokemon bar Glacia.
Sidney has Mega Absol
Pheobe has Mega Baette/Sableye
Glacia has nothing (Although she could inherit Mega Abomasnow, considering GameFreak might want to give her more Pokemon than just two Glalie's, two Sealo's and a Walrein)
Drake has Mega Altaria or Mega Salamence
and Steven Stone has Mega Metagross
Assuming Glacia gets a Hoenn Mega, it's not hard to assume that the second (harder) run of the Elite Four might involve all the Elite Four getting Megas.