Tbh though the next game being called "Mario Kart 9" feels pretty unlikely even ignoring Tour.
Beyond how they've basically said in the past that they don't intend for the numbering thing to continue, this game just has a kind of reboot vibe that would lend itself much better to a subtitle.
Most people who play Mario Kart don't really care what a "mainline" title is, and I would imagine most would be confused from going from Mario Kart 8 to Mario Kart 10 even when Mario Kart Tour exists. Home Circuit and Arcade GP also exist, and don't think ppl consider those sequels to prev games
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SMK, MK64: console
MKSC, MKDD: subtitle
MKDS, MKWii: console
MK7, MK8: number
MKTour: subtitle
cause the last one was Mario Kart 8D
Beyond how they've basically said in the past that they don't intend for the numbering thing to continue, this game just has a kind of reboot vibe that would lend itself much better to a subtitle.
So many games go to the next numbered entry even if multiple games have released between the last one with a number
(Like if Game came out, Game 2, Game the return, Game the origin, than Game 3).