After seeing a recent video by MandJTV, he brought up some interesting points regarding Mega Evolutions.
Before X and Y were released, a good chunk of old Pokemon shown in the previews got Mega Evolutions in those two games. (Such as Lucario, Tyranitar, Gardevoir, and Gengar.) Although, there were a fair share of Pokemon shown in the previews that didn't receive Megas in that generation, such as Tauros, Hariyama, Snorlax, and Zoroark. Audino and Salamence are exempt from the list, since they did eventually receive Mega Evolutions in ORAS.
With Sun and Moon's previews, some new Pokemon can be seen fighting some older Pokemon that already have Mega Evolutions beforehand. The older Pokemon shown in the trailers that did receive Megas were Salamence, Metagross, Gengar, Kangaskhan, Slowbro, and Garchomp.
If history were to repeat itself again this time around, we might have some new candidates eligible for Mega Evolutions. The most potential candidates showcased fighting some new Pokemon are Flygon, Milotic, Dragonite, Fearow, Pangoro, and Krookodile. They have not yet received Megas at this point in time. A recent Japanese trailer showing off Rockruff and Komala showed both Pokemon fighting Arcanine and Electabuzz respectively. This may not say much, but it might hint that some more Pokemon from the Kanto Region might just get Mega Evolutions to celebrate a big milestone, such as the 20th anniversary of the whole franchise. To me, this would hint at what Egg Groups both Pokemon will be a part of.
Although, not all of them will get Megas at the end of the day, but from what X and Y showed off beforehand, we might be seeing a similar pattern being repeated this time around.