Ash actually had more to say when he learned about M-Ampharos ("Wow! So Ampharos can Mega Evolve too? Woah! How cool is that?") than he did when Sawyer mentioned M-Sceptile ("So Sceptile can Mega Evolve?!") This is going by the dub, of course.
The one on the poster could be Ash's, but I don't think his reaction to Sawyer mentioning M-Sceptile should be treated as evidence/foreshadowing/etc. Ash & co. are just surprised/interested whenever they learn about a new Mega Evo.
The difference though is Ash actually has a Sceptile.
Are we really to expect if Sawyer mentioned any other mega evolution unrelated to Ash, Ash would've still been impressed, or maybe would the line being given to Serena, Clemont, or Bonnie.
Because think of Mega Blaziken, Ash was impressed, but he didn't imply it was super amazing, in the sense he had one. Mega Ampharos, not sure what happened there because as far as I'm aware we haven't seen it. So Ash may have just been impressed.
Plus given the context of what Ash said, I think Ash just wasn't given a very big line in regards to Sceptile. I think the raw said something like "Sceptile can mega evolve?" in a way that to me, implied he was very interested to know that.
I know Charizard, but keep in mind Ash doesn't know it can mega evolve, but does know Sceptile can. So I feel like the big main mega evolution for Ash could indeed be Sceptile, while Charizard will merely be introduce Ash into mega evolution.
I feel as if maybe Ash's Mega Charizard Y will be only temporary for temporary purposes, and has to give up Charizardite Y to Professor Sycamore, but sometime later picks up Sceptilite and that's why Ash would have mega Sceptile.
Of course the concept of Mega Greninja is not to be ignored. We just have no idea.
Anyone else see a digimon theme going on with greninja.
I just hope they dont decide after mega evolution to start merging humans and pokemon.
That creates a paradox, since Ash is ALSO on the poster.
I think at worst or best Greninja has protean and absorbs some of Ash's DNA and perhaps composition of Ash's clothing while being exposed to some form of energy and adopts traits of Ash, not fusion.
So kind of like, I know this a touchy subject, but just for comparative sake, the Hiroshima bombs, and how silhouettes were burned into walls of the people when the bomb exploded.
I think Ash might block Greninja when a sort of energy hits and transfers, DNA and certain aspects to Greninja and Greninja adopts this new form.
I'm almost positive its not fusion like Digimon, or Dragonball Z.