They never restrict you on dimension (reasons include using an only 3D engine like unreal engine), its not fun if they have to condition you on the art style and themes are usually more abstract than 'farm'. Take the previous two jams. The themes were 'Stronger Together' and 'Rewind'. With the theme Rewind, people could have done things like rewinding when you die, re-watching your progress and some people even made a YouTube rewind game. Farm isn't too great of a theme for this reason. And also, with the '2D' limitation, it basically locks you into a farming based game. With 3D an 'escape' sort of game could have been made (even with pixel graphics, there are a lot of voxel art tools).
