I'm starting to become an active Garchomp supporter, cause it's a Pokemon with timeless popularity, and damn is he just cool and would make for a nice Smash fighter too. Something wicked and vicious looking, something different than the rest. Garchomp is just about a staple now in so many Pokemon games that don't follow the traditional gameplay, as Pokken and Pokemon Unite, it's only fitting I'd say.
Others I also think would be cool are Snorlax, and hear me out, he'd be a heavy weight Jigglypuff semi clone. Pound = Mega Punch, Rest = a healing move that's also Rest, Rollout can actually stay, and Up B could be something like Body Slam, think a cross between Dedede's Up B and Bowser's Down B.
Then there's Raichu whom I think would be cool as a Pikachu semi clone based around how Pikachu fought in Smash 64, maybe a bit slower , maybe a tipper mechanic on its tail too, and a Neutral B attack you can charge up (which is a thing I want for Pikachu too).
Blaziken is also a favorite that always seems to pop up when Smashers talk of Pokemon clones. Based around C.Falcon is a great concept yeah, but I agree with this one poster of earlier than a "Frankenstein" situation would be cool for Blaziken, pulling moves from different characters as Falcon, Incineroar, Charizard, you name it. It would fit, so why not?
Outside of these semi clones and Garchomp I have no huge preferences. Though I must say, if they include Blastoise for the "rivalry" with Charizard, and Mew for it's rivalry with Mewtwo, that would be awesome. That would make for wicked reveal trailers I always felt. Remember one of the first Pokemon Stadium commercials? That's how it would be. Cool as ****.
My age is showing here talking about these old 'mons, but I don't care. They're Nintendo characters after all and many have lasting popularity, and that's what counts. There's gotta be a time when adding the newest flavor of the month is just not gonna cut it, unless they're Lucario / Greninja levels of popular.