Basically what I do is keep track of what Pokemon Gamefreak gives Sakurai permission to use in Smash. I don't know anything about what non-Pokemon characters are in. The fighters at least should be officially revealed during tomorrow's direct.

-A lot of Pokemon are approved for background use. This usually means trophies, or maybe appearing on a stage but not interacting with characters. All of the Pokemon from Pokefloats are listed, so that might mean it's back. If so, I don't know if it's changed at all.
-Another list is Pokemon approved to interact with the player, either as part of the stage or being a Pokeball Pokemon. Most Gen 7 Pokemon were allowed to be Pokeball Pokemon, but that doesn't mean Sakurai will actually put them in the game.
-The pokeball Pokemon that I know for certain are Decidueye, Midday and Midnight Lycanroc, Charjabug, Kanto Exeggutor, and the base forms of each Kanto starter.
-The master ball Pokemon I know for certain are Cosmog, Naganadel, and Necrozma.
-Aether Paradise will be a stage.
-Cloyster will be the next fully unique fighter to be revealed.
-Zeroara will be an echo fighter of Lucario.