I knew I wouldn't have enough time, but I also know I can make quick simple games in a short amount of time. So of course, I decided to make an open world multiplayer roguelike deck builder, that should be quick and easy! I am not certain if I wrote a single line of code, but I did throw a lot of things in to Unity and made a mess. So, there is no playable game, but at least I can put up a screenshot.
I do have a theory that game jams are not always about making useful things, but just taking ideas and throwing them at the wall and seeing what happens. It's art, as they say. So, there you are.