I've been wondering this for a while. I think it could be a great way to revive interest in the series for those who are starting to run out of fuel for it having been playing since the Wii U, whilst not getting in the way of MK8's seemingly unstoppable sales.
If not, then I definitely want Super Mario Kart 30 year anniversary edition (with added racers, online and 16-bit remakes of newer tracks).
But a Mario Kart Maker would be outstanding if it's at all possible to the necessary Nintendo standard. My guess palettes would be SNES, N64 and MK8 at the bare minimum. GameCube and Wii would be a nice bonus, and GBA, well, whatever :/