Nintendo has a particularly cruel, vague tease for fans eagerly awaiting the next mainline Mario game: just watch future Nintendo Directs.
Mario creator Shigeru Miyamoto was interviewed by Variety ahead of the release of the Super Mario Bros. Movie. To close the interview, Variety asked "Now that we have the movie, and Super Nintendo World amusement park, when can we expect a new Mario game?"
With a big ol' smile, Miyamoto responded: "Well, all I can say is please stay tuned for future Nintendo Directs."
There you have it, folks! You should watch Nintendo Directs if you want to see the announcement of upcoming games in the publisher's most popular franchises. I'm no insider and I don't have an uncle who works for Nintendo, but I feel like I probably could've told you that a new Mario game will someday be announced at a future Nintendo Direct even without a Miyamoto quote confirming it.
Jokes aside, it is legitimately very unusual for Nintendo to even half-seriously acknowledge in-development products, especially when it comes to something as big as a mainline Mario game. There have been rumors of a new 2D entry since last year, and fans are growing desperate. It's been nearly six years since the release of Super Mario Odyssey, and even innocuous marketing emails have been enough to raise expectations to unreasonable levels.
When will that new game actually hit? I think for now, the only reasonable thing we can actually say is 'someday.'
If you're looking for upcoming Switch games that have actually been announced, you can follow that link.