Adding to these previous rumors is a new anonymous rumor that claims Nintendo is planning to mix things up with the next installment in the long-running and popular series. According to this rumor, Mario Kart 9 will tap into Super Smash Bros. Ultimate in the sense that it will be a crossover game featuring multiple Nintendo franchises.

This is the major detail divulged, but the rumor also notes the game is near completion and ready to ship this year, possibly during Q3, which is to say, this summer.
Beyond this, the rumor notes of a new "tackle" button on RundL and a "boost" button assigned to shaking. The rumor also mentions a few characters that will appear, such as Breath of the Wild's Zelda, Mario, Koopa, and Ice Climbers. And that's about where the details end, at least of the salient variety.

QUELLE und TEXT: https://comicbook.com/gaming/news/mario-kart-9-nintendo-switch-lite-super-smash-bros-ultimate-rumor/

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