I think it's a possibility but I'm not sure. The next Mario Kart will most likely release on Nintendo's next handheld, so we're talking at least 2-3 years. If Nintendo acquires SEGA or Capcom in that timespan, then maybe we'll see new characters although they will technically be first-party by then. I think that the Mario Kart series will really need to be rebooted for the next game if it wants to continue to sell as many copies as it has been. As for any other possibile third-parties, I don't know any since most just don't fit into Nintendo's atmosphere/aren't that popular or unique.