No Discord at the moment, but that's a good idea - maybe we'll set one up. If so we'll let everyone know! Though it might be something we just plan for next year's jam.

As for the rules: the game must in some way involve a work that entered the public domain this year. That doesn't necessarily mean you need to directly use the work itself, but it still needs to play a role in the game somehow. Either way, you can still incorporate lots of your own original art/music/etc. Maybe you're retelling a story from a 1926 novel or film with your own twist, or mashing several together, but all the actual material (art, music, etc.) is your own! Or, conversely, maybe you're using lots of art from 1926 as graphics to make a completely new game that has nothing to do with the original context of the art - maybe even the 1926 works are just background imagery. Or somewhere in between! As long as it incorporates at least one newly-public-domain work, somehow. Note that this doesn't mean the game must be set in 1926 - though that's certainly an option.