Pokemon has a large amount of characters already, 7 unique characters (9 counting Squirtle, Ivysaur, Charizard as all different characters) for comparison the Super Mario series has 7 unique characters (and 1 echo).
Another thing to consider is that of all the returning franchises in the Smash series Pokemon is the one with the most cuts. Mewtwo, Pichu, Squirtle and Ivysaur have all been cut at some point in the series. Now that everyone is back, Pokemon fans have more options than ever. I think a new pokemon rep would be suitable for DLC but, not for the base game (in my opinion).
Its more than likely an inevitability we are getting a Pokemon base game, unless Sakurai explicitly sticks with the former
fan service only from here on forward statement. However...
There are so many types that are unused and so much move potential still untapped that it seems a waste to not add one.
This can be said with just about any series we have. Especially Fire Emblem with how there are so many other classes of characters that go unused in Smash. Yet I don't think we need another Fire Emblem character (
Despite how much I enjoy them). Lyn is a very popular character, though it might be overselling the swordsman motif for most people. There are characters like Hector, Ephraim, Camilla, Tharja, and so on that are all stylistically different weapon wise. (
Say what you will about waifus or whatever. But these are popular character choices with very strong moveset potential to be different.)
There are a bunch of bounty hunters in Metroid that go unnoticed. Two favorites in particular being Sylux and Rundas.
There are plenty of fan favorite Mario characters that are still being sidelined. Geno, Waluigi, Captain Toad, take your pick.
Donkey Kong has many characters that are ignored almost entirely. Any one of the other Kongs specifically. Especially Dixie, Funky, Cranky or Lanky.
Wario's characters from whatever series you want to pull from rather it be Land or Ware both are pretty ignored despite what they offer. You got characters like 9-Volt, Jimmy T., Ashley, and on the other side you have Captain Syrup.
I could go on about each individual series for a long time. But these are just some off the top of my head being an outsider to most of these series.