I think that Im going to stay away from enforced theme choices in the future, and stick to common design constraints. Kind of like the "treasure hunt jam". It wasn't very prescriptive except that graphics were not allowed.
I'm glad we had Halloween and Christmas jams, but the Christmas jam in particular was kind of tough to adhere to the theme.
If there is a themed based jam in the future it'll probably be super vague. Something like "mythology", or "sci-fi".