So about Pokemon, to be honest, I think Pokemon has a say in having 2-3 Newcomers for its franchise in the Next Smash. It doesn't all have to be in the base game
1 Newcomer for Gen 8
1 Newcomer for Gen 9
1 Newcomer for Gen 10, Likely DLC.
I think going into the next Smash game, I think DLC is probably going to be planned from the Start at the same time as base game, and have a bigger focus in this Next Smash game than the others, Seeing how Smash DLC has been successful for 2 Games in a Row, with Ultimate getting a Whole other pass as well as other DLC in other Nintendo games
So I don't think they need to do the thing where they reserve a spot for a Pokemon that to us, don't even exist yet. That's what DLC is for, and I think people will buy it anyway, People love Pokemon. Sword and Shield, as well as Scarlet and Violet have both sold 20+ Million Copies. They are also among the most sold games in Japan currently.
I'm also on the train that we will keep most, if not all of 3rd party characters, with the Newcomer list leaning more 3rd party than people might expect, so I think having some Nintendo characters that people would buy (So Nintendo can keep all the profit from that DLC Pack) would be vital
So I think they could look at both Gen 8 and Gen 9 and pick a Pokemon based on what people have shown to actually like for the Base Roster
Personally from what I've researched, I would go with Dragapult. It's a Ghost Dragon Pokemon that shoots Missiles, and I believe won the poll for Most popular Gen 8 Pokemon, and still is popular in Gen 9. That sounds cool to me even as someone that currently doesn't play Pokemon, and Pokemon fans like it. There's other options like Toxicrity from Gen 8, and I've also seen Tinkatron and Meowscarda for Gen 9. They could even go back and do Decidueye based on it's Arceus form, though I doubt that option