Nintendo of America has announced on Twitter that a new mobile app called Mario Kart Tour is now in development, but you shouldn't hold your breath though, since the launch window is the "fiscal year ending in March 2019," which means it's coming between April 2018 and the end of March 2019.
The short tweet doesn't mention if this app is a full mobile Mario Kart game or whether it's a sort of a companion app to an existing Mario Kart game in a similar fashion to what Splatoon 2 has with it's own app. To be just an app for Mario Kart 8 Deluxe seems a bit late at this point, though, so maybe Mario Kart Tour is indeed a new mobile game from Nintendo.
As you know, at this point mobile games aren't a new thing for Nintendo, since they've already released Super Mario Run and Animal Crossing: Pocket Camp for Android and iOS devices, and back in May 2017 it was also announced that a mobile The Legend of Zelda game is in development as well.
Do you think this game will be a mobile Mario Kart?