Sakurai never puts the "obvious" pokemon pick in Smash, Decidueye is not likely at all to me, but people can dream i guess
I mean, Lucario, Charizard, (not apart of Red's moveset but aside from that) and Mewtwo are "obvious" picks in a lot of people's eyes, including mine. There's some outliers such as with Jigglypuff, Pichu, Greninja, and Red's other ones such as Ivysaur and Squirtle.
Like with the former, they were fan favorites and popular Pokemon that were being pushed by the Pokemon Company and Game Freak. Greninja is especially the latter, but by the time X and Y came out he became
very popular with both the competitive and casual crowd.
I think this time around, there's a lot of different possibilities to consider given both ORAS and Sun/Moon are obviously prominent, one being around the time the project plan was finalized and the other marking the 20th anniversary of the franchise.
We're literally in the same place we were with Smash 4, but instead of people arguing about which Gen 5 character may make it in, there's a large swath to consider. And if we believe Verge's claims, which I don't think we should, it narrows it down to one gen but still leaves a lot to speculate upon.
Edit: Talking about Verge, he's flipped flopped so many times on MvC:I and SFV, that I would not take anything he says at face value. Also with the fact he has a seemingly big ego and doesn't hesitate to drop info and with Sakurai's comments on how a large portion of employees didn't know about the roster given how secretive they've been, I wouldn't take anything thrown around seriously until we see definitive proof.