hi sorry for the late reply! yes go for it you can start building anytime; you can even just keep working on a game you made before!
there is no required theme (there will be an optional one just for brainstorming purposes if u want, but it's not gonna be judged), there's no set type of game (but web builds would be nice), and we are mostly looking for vibes & having a good time, but if you want to do well i would suggest adding millie somewhere :)