Super Smash Bros. Ultimate DLC: 7 Pokémon We Need To See
1. A Pokémon From Generation 8
This is the most likely Pokémon to appear in Smash Bros Ultimate as DLC, being Pokémon generation 8 is arriving towards the end of 2019.
Smash Bros Ultimate will likely feature a starter or mascot for generation 8 as a playable fighter to advertise the new games. Or at least, that would make the most sense.
Smash Bros has almost always had Pokémon from the latest Pokémon games featured as playable fighters, with Pikachu and Jigglypuff in the original, Pichu in Melee, Lucario in Brawl and Greninja in Super Smash Bros. for Nintendo 3DS and Wii U.
So with a new Pokémon game coming out in a time period where they are releasing DLC for Smash Bros Ultimate, I doubt they'll miss the chance to feature a new Pokémon in Smash.