Concept: Undertale kart racing game
All the friends from UT are playable with more rare ones (ok, Asriel and Chara) unlockable. Each character has their own special vehicles, like Papyrus's red hot rod or a limousine for MTT.
The game would have courses set in overarching thematic worlds- everything from the Ruins to New Home to the surface- but with special courses per location, like a Snowdin Town based course with an impromptu detour through Grillbys, or a course through the castle up to Asgores throne room.
Each major boss could have their own challenge courses, like Papyrus's Perplexing Pasta Puzzle Palooza, which I picture a lot like Waluigi Pinball... And the last unlockable boss course is just called "sans." And everyone's expecting an epic Megalovania track that's tough as nails but it's just a straight course in a white void with no items or obstacles