Well, tbh this was one of the first theories I came up with when I saw the new Sun and Moon trailers.
I would love the idea of getting Mega Flygon, Milotic and Dragonite as multiple posters (you know, people who post, not actual posters lol) have said, even though (as Pinkie-Dawn brought up) we would get Dragoniteite, and I would love it.{XD}
Personally, I wouldn't mind a Mega Pangoro, but I'm just worrying design-wise. Likewise, Mega Krookodile wouldn't be too bad either, as long as they make a nice looking design for it. Mega Fearow is something I would see people who draw Mega Evolution fan-art make, but I don't think I'd like it that much.
Anyway, that wasn't the point of my post. I actually wanted to bring up the fact that it looks like only Pangoro and Krookodile actually seem to have a trainer. This could mean that the battle that was shown in the trailer is an actual in-game trainer battle, while the others were pre-programmed wild encounters that are not going to take place in the actual game. Or the current engine just doesn't support double wild encounters, in that case you can scrap that from the 'possible features'-list.
Also, to continue with what destinedjagold said, I think the crystals may have something to do with Synergy Burst as we saw it in Pokkén Tournament. Mega Evolutions were a type of Synergy Burst in that game, which could possibly mean that there isn't a reason for them to stop introducing Mega Evolutions. Maybe I should just make an entire new thread about this though. I'll do, one day...