Would you say that creating a fangame would be reasonable? For example, making a sonic game by creating your own assets, based on your own original story, but with any twists/improvements that you care to add. Obviously the game couldn't be sold or monetized in any fashion. There are a plethora of Sonic/Mario fangames that exist, and they often have their own assets that were created for those games
If you make a fan game and it’s clearly marked as such and you release it for free (which is kinda necessary for a jam entry), I don’t think anyone will bother you. At the worst, if your game was amazing and went viral, and used some Nintendo or Bethesda IP, you might get a Cease and Desist letter from their lawyers about it. SEGA seems to like fan projects.