When Nintendo first unveiled Nintendo Switch last month, the reveal trailer showed what appears to be a new version of Mario Kart 8. While the visuals matched the Wii U game perfectly, the brief clip showed two new additions: King Boo and the ability to hold two items at once. Many have speculated that the Switch version of the game will be an enhanced port with a mixture of new and old content, and according to the latest rumor, that's exactly the case.
According to tipster Emily Rogers (via "sources close to Nintendo"), the Mario Kart game on Nintendo Switch will include all of the tracks and playable characters from Mario Kart 8, including all DLC, and it will also feature a healthy dose of new goodies. This includes brand new tracks, new playable characters (like King Boo), and a new and improved battle mode. Rogers states that Mario Kart 8 has wrapped up development, and Nintendo is planning to launch it within three months of Nintendo Switch's debut.
Please note that this is a rumor, and none of this information is officially confirmed by Nintendo at this time. That said, all of this seems extremely plausible, and I'm inclined to believe it.
Source: Emily Rogers