Now that I took part in quite the number of gamejam, I think a lot more about what make a good theme. Still no solution. However, I've noticed that I judge themes more by thinking about the game testing phase than the creation phase. Which theme won't give multiple copies of the same game, which theme is too complicated to apply to a jam with so many people including many newbies. I find it interesting to have gone from an individualistic vision of my theme choice to a collective one, precisely by spending time to note and analyze the games of the others.