Fair enough, but it still seems odd for the reveals to be so late on a Friday night.
As for what the reveals could be...something that has stood out to me over the past few months is that in all of the trailers for ORAS, the (fully evolved) Pokemon featured in them have either already been revealed to have a Mega Evolution or were later confirmed to get one. As in, some of them showed up in trailers and then were later revealed to have a Mega Evolution.
For example, in the trailer where Mega Altaria, Salamence and Lopunny are revealed, Altaria battles a Shiftry, Salamence fights a Torkoal and Lopunny is against a Sableye (which already has a confirmed Mega). In the most recent trailer for Mega Camerupt, Sharpedo, Gallade as well as the reveal for the 'harsh' weather, Groudon fights a Milotic, Kyogre battles a Slowking, Sharpedo fights a Dusknoir and Gallade is against a Shiftry again. Rayquaza also battles a Salamence (which is another confirmed Mega).
Before this, all new Mega Evolution reveals were shown battling against the Mega Evolved starters. That said, I'm pretty sure Sableye was in the trailer that the Mega Starters were revealed, and I remember seeing Ludicolo and Slowbro in one of the videos as well, but I can't seem to find it at the moment. I also recall Altaria making an appearance earlier when contests were revealed.
My point is (and I could very easily be reading too much into this), that maybe Nintendo are hinting at which Pokemon are going to get Mega Evolutions by showing them in the trailers long before their Mega Evolutions get announced? Which would point to Shiftry, Torkoal, Milotic, Slowking and Dusknoir at the very least, all of which make sense to receive Mega forms anyway.
