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The fact that Legends is a staple is pretty obvious, and it's clearly too new for a new game announcement from Legends, which makes me think they're bound to announce DLC, especially with what's in the game files.
What's more in flux is the second component of all of this, it's not entirely clear how Gamefreak will move forward supporting the multiplayer scene. A stadium game makes a lot of sense, another story based mainline style game seems like a lot when Gamefreak only just dropped Legends and BDSP also came out and will have Legends DLC on the way, but on the flipside I wonder about the business viability of a Stadium game, that's theoretically an inability to resell you the Pokemon multiplayer experience again in the Switch's lifespan, it also begs the question of what a "you're supposed to do multiplayer here" game does to the life of BDSP and SWSH but I suppose that was the case with ORAS and XY and others as well.
I also think it's pretty likely they could announce some sort of substantial side game that would be a hit with people, like New Snap was. Maybe a new Mystery Dungeon, Pinball, Rangers, Conquest, Pokken, Detective Pikachu, or something new.
I think that the announcement on Sunday will be Legends DLC and either a new Stadium game or some sort of substantial 60 dollar side game ala New Snap.