A theme would help to ensure people do not get a head start in building their game. As in, the theme would be announced at the beginning of the jam. Not like it's a competition though
I agree, I don't think there's any concern of people getting a head start when there's no judging, no prize, and the goal is just to learn a little Godot :D
We all win - and if someone starts early, they just get to learn more!
