I would really like to see a return of the Yoshi's Island series on the Nintendo Switch, and by that I mean a game more like the original and Yoshi's Island Ds. Add more babies with specific abilities/powers, Yoshi vehicle transformations, colorful graphics that look like they are powered by mode seven, and new eggs to throw at baddies.
Additionally it would be really cool if they made a new Pokemon game like Pokemon Colosseum, where you get to go on a full 3d adventure in a different land doing things that you normally can't do in regular Pokemon games, like fighting shadowy versions of legendary Pokemon. A game like Battle Revolution would be nice too.
Finally, like a bunch of other people have already mentioned, a new Metroid game. Particularly one that take place after Fusion, since it would be nice for the Metroid timeline to move forward rather than just continuously adding more stuff in-between already existing games.