Oh, they're almost 100% bringing Mega Evolutions back for this game. We saw the Mega Evolution symbol in the trailer and everything. We're likely getting a few new ones, too. Probably not a lot, because Legends Arceus only got a handful of new Pokemon, about 30. And they'll probably split the "New Pokemon Design Budget" between Mega Evolutions and new Regional Forms. So, I'd only bet on only about a dozen new Mega Evolutions, if they also give us another dozen or so new regional forms, too.
...I am bracing myself for the possibility that they nerf Mega Evolution, though. I could totally see Gamefreak changing the rules so that instead of lasting the entire battle, Megas only lasts 3 turns like Dynamax or something. Gamefreak does seem to have some sort of problem with Megas, given the Sun and Moon Pokedex entries getting kind of oddly dark. I don't want them to do this, but I am preparing myself for the possibility of them doing that.
I am hoping that Legends Z-A gives us some clarification on what actually is the "source" of Mega Evolution, though. All the other "gimmicks" have pretty obvious "source" Pokemon that are responsible for them. Z Moves come from Necrozma. Dynamax comes from Eternatus. Terastallization comes from Terapagos. But, Megas have been slightly unclear.
Deoxys is the most likely "source" Pokemon for Mega Evolution. Given the way the symbol for Mega Evolution is a DNA strand (for DEOXYribonucleic acid). And we see that the Mega Stones seem to come from outer space, like Deoxys arrives in a meteor in ORAS's Delta Episode. But, Deoxys doesn't USE Mega Evolution in the same way the other "source" Pokemon do. So, it would make sense if Deoxys was one of the Pokemon who got a new Mega Evolution.