I love Mewtwo, and do want him back, but this Hoenn confirmation is enormous. I'd love 7 poke slots with Mewtwo, Jiggs and Sceptile, or even six with Jiggs cut, but I do think 6 is a little more likely and Jiggs being cut is a little less likely. So as I see it, most likely it's either Mewtwo or Sceptile. I never thought Mewtwo was a lock by any means, but a pretty damn good shot until today; this burned him really hard.
Here is a fact that needs to never, ever be forgotten about Sakurai's roster choices: It's not dictated by fan popularity. A hand-full of picks will, yes, be prioritized by popular demand, but the majority of the selection is what Sakurai and Sakurai alone wants: who matches his vision for the game, who compliments the rest of the roster, etc. There are two main reasons he may pick Mewtwo over Sceptile, popularity (see above) or relevance, and Sceptile just got a turbo-shot of that injected straight into his temple today.
Now Sceptile versus Blaziken is a different matter. You'd be out of your mind if you don't believe Sceptile and Swampert are getting Mega-evolutions. Unlike Blaziken, theirs are going to be brand new, fresh out of the package. And yes, type does matter. It represents the core way in which pokemon are diverse, visually and functionally from one another. Having more pokemon, or characters at all, that use fire as the basis of their attacks, especially a second pokemon, is clearly unappealing from a design stand-point. What other character is going to be using plant-based attacks? No one, certainly not Ivysaur. So I submit yes, Sceptile's type, and by extension his overall design, is much more attractive than is Blaziken's. And then consider the fact that starter Pokemon have always been a phenomenon connected by one major thing through all of these games: 1 grass, 1 fire, 1 water. It represents the most core and defining feature of the Pokemon games' battle system: the complex "rock paper scissors" relationship between the types. And as far as pokemon culture and fandom goes, I've always seen an attachment in people to the starter type. There are people prejudiced toward the grass starter, the water starter, fire starter. It's like little 'camps'. People will say such things as "I always pick the fire starter but this generation's fire type sucks ***, I might have to go with Froakie." Or better yet, "I really like Froakie, he's my favorite of the three but I always pick the fire starter, so I'm going to still have to go with Fennekin." I think it'd be foolish to believe that Sakurai would ignore the 'starter type trinity'; he understands pokemon too well to do that. This thread's existence is proof in and of itself that people naturally see it that way, that it makes sense to people.
"We have Charizard and Greninja, where is the grass starter!"