As it currently stands Mario Kart 8: Deluxe is quite possibly the most popular racing title on the Nintendo Switch right now. Since its release back in April 2017 and has managed to sell over 11 million copies worldwide. The original version of Mario Kart 8 was released back in May 2014 for the Nintendo Wii-U and since then many people have wondered whether or not there would ever be a Mario Kart 9. Back in 2017, there was some speculation that we would have seen a Mario Kart game sometime in 2019. However there has been no announcements from Nintendo whatsoever about the release of a Mario Kart 9 game for this year.
Seeing that Mario Kart 8: Deluxe is still very popular it would not be surprising if Mario Kart 9 does not appear on the Nintendo Switch until 2020. Hopefully, this year we will see some type of reveal trailer of Mario Kart 9 from Nintendo but at the same time it seems unlikely especially since we have yet to see one for Metroid Prime 4 a game that was expected to appear at E3 2018 and did not.