The only problems with Mega Evolution Mewtwo vs Z-Move Mew are that a) We've already got several Mew promotions (with the Z-Crystal in SM/USUM and the Lets Go Mew) so idk how they would be able to create the inevitable promotion and make it special considering that Mew is already handed out in both games. Especially considering that I doubt Z-Moves will exist in Lets Go, and I don't think that they're going to give any more event Pokemon to SM/USUM after it comes out (unless it's a SHINY Mew, which I would be pretty happy with lol) and b) I know they've worked around mega evolutions occurring without a trainer, but Z-Moves without a trainer? Doesn't seem that easy to explain, especially considering that they're established mechanics, not like mega evolution in the Genesect movie.
Then again, hoping that it's set in the main timeline and the hyped reunion movie that episode 20 was supposed to be, not to mention a sequel to the Mewtwo movie and Mewtwo Returns. I'm just speculating, but I'm hoping that this is a movie where the build up is in the anime, like the original Mewtwo movie. Here's how I think the plot should go: After his (inevitable) scheme with Necrozma is foiled by Ash, Giovanni can actually find Mewtwo again in Johto, control it in some way or another, mega evolve it, and send it to wreak havoc. Because this is a worldwide catastrophe, all the companions/gym leaders/whomever else unite with Ash to finally bring down Giovanni once and for all. Naturally, Giovanni/Mega Mewtwo X destroy them, but Mew and the other clonemons from movie one show up in the nick of time to save them. Mew decides to work with Ash, who, naturally, has a Z-Ring from his island trials (thus solving point B from above). We get a PROPER Ash vs Giovanni battle (finally, after 20 years), and some sort of resolution/ending tying up all the knots left over from this movie and/or earlier seasons of the anime.
That total speculation which will probably evolve into a fanfiction, but I'm hoping that this is somewhat close to what we actually get.
I firmly believe that the "evolution" part of the title doesn't refer to Mewtwo's power, but rather some sort of CHARACTER evolution. I see no reason to rehash the Mewtwo movie (which is universally considered to be one of the best Pokemon movies), since the character of Mewtwo is already very well-developed, and the writers aren't going to create a remake when people can just stay at home and watch the (probably better because nostalgia) original; they will lose quite a bit of money if they do this, I think (then again, iirc movie 20 did that with episode 1 and look at how well it did, so this point is probably out the window). It would be silly to go back and recreate a Mewtwo that hates all of humanity, and has to be retaught what he learned in movie 1. It just seems to me like they would be going backwards. Because as I mentioned earlier, movie 20 did that with the first episode, but that was 20 minutes of the movie, and the rest was all new content. This would be an ENTIRE movie that would be remade, which just does not seem feasible, since the fandom would probably flip out if there was new content destroying the nostalgia of the first movie, but also be upset if the movie was a carbon copy of the original. There is a fine line, and I think if the writers went down the remake path, they would not be able to succeed creating a good movie that is a happy medium between the two.