A lot of people keep mentioning Mario Kart 9, but I am not convinced Nintendo will release Mario Kart 9 on the Switch. I mean, why would they need to? Just look how Mario Kart 8D is selling. Why release a new game that is just going to eat into the sales of the old game that is selling like no tomorrow?
Mario Kart 8D is a lot like GTA 5. Technically GTA 5 was a PS3 game, but it also is the best selling game on the PS4. Why would they release GTA 6? (So far they haven't.) Since GTA 5 keeps selling there is no reason to release GTA 6. By the same token, since Mario Kart 8D keeps selling, there is no reason to release Mario Kart 9. They could just keep developing it, and then release it as a launch title for the Switch's successor.