Here's my out-there prediction for the first year: a game using the "Smash" brand that isn't a fighting game.
To explain, after Smash Bros Ultimate finished development, I don't think Sakurai would have been particularly eager to jump back into developing another Smash Bros right away. Three of the last four games he's worked on have been Smash Bros (or four of five if you count Wii U and 3DS separately), and he had been working on the franchise continuously for close to a decade by the time Ultimate's DLC finished.
From Nintendo's point of view, they probably don't want to strain their relationship with Sakurai either by trying to strong-arm him into developing another Smash Bros game immediately, or making another game in the series without him. So, they let him pitch some other game ideas, publishing (and perhaps assisting development of) whatever they like the look of, possibly with the agreement that another Smash Bros would follow.
Watching Sakurai's YouTube videos on the design process of his games, there's a clear theme of him taking an existing genre and trying to put his own spin on it, challenging some of the conventions of the genre. Smash Bros was his take on the fighting game genre, Meteos his take on the falling-brick puzzle game, and Kid Icarus was his take on the third person shooter. With both his games for Nintendo and other outside studios you'll also notice that the core idea of the game typically appears before they decide which franchise it will fit into, with Nintendo and the developers trying to find a good match for one of their existing franchises.
So, Sakurai pitches a new game idea to Nintendo, let's say a new type of battle royale game for the sake of the example, and together with Nintendo they have to decide what franchise would best fit this new idea. Why not use Smash? It's a franchise already associated with Sakurai, and he's clearly demonstrated an ability to handle crossover games extremely well. From Nintendo's point of view, which do you think would sell better, Star Fox Royale or Smash Royale?
Of course this only really fits if he's working on a multiplayer game, as I can't see a crossover between so many franchises work well for a single-player story based game. If it is a multiplayer game, though, then it seems to me that treating it as an expansion of the Smash franchise would make a lot of sense.