What about Roblox games like Pressure, Deepwoken, DOORS, or some other common Roblox game? Those have some pretty good soundtracks.
Viewing post in Future Themes for Jam Suggestion Thread
I think I might group for genre with Roblox themes. From what I've seen, there seems to be a pretty big demand for Roblox horror music from indie devs and I think it could be a good theme for musicians to practice horror/thriller soundtracks but also it could make a good collaborative non-commercial event for creating music that can be used by game devs (but it will be opt-in and not a requirement for the jam)