Sakurai is usually the one choosing which Pokémon to rep isn't he? I recall he said that Decidueye almost became the playable Gen 7 rep. If Nintendo decides on "new Pokémon" I could see this being the one candidate that Sakurai may have more of a choice on the matter of, since he values moveset variety. I think any reason to assume starters would be the front runners is because Pokémon has been kind of flunking for a while now with pseudo-legendaries or any other standouts. Recall that there was a time when very few expected Greninja to be in Smash 4. A lot of people expected an Unova rep like Zoroark. Greninja's inclusion came as a massive swerve to many since starters had never been repped at that point, and Kanto Pokémon Trainer was arguably the exception in service of a gimmick. Cinderace is arguably a good frontrunner, but I'm left wondering if Sakurai thinks the character offers much new, the same way Incineroar had a uniquely grappler-type move set.
I was thinking with Toxtricity they could implement a form change mechanic. At first I thought if he got into Smash then his Amped/Low-Key forms would be alternate costumes, but given that both forms have a different move tree it could make for an interesting gimmick to swap between two sets of specials (his G-Max also combines both forms). I reckon it'd also be easier to implement all the posture changes.