Can I suggest that perhaps there be multiple themes? As in, people get to pick their own grouping from a set of two or three of the popular themes?
I’ve participated in lots of jams over the years, and also helped to organize a local jam a few times. My conclusion about themes is this: they can never be good. Everyone has their own opinions about why they like a theme, so whatever you pick, someone will be unhappy. Crowd sourcing is an appealing but ultimately ineffectual solution.
I personally really dislike themes that feel mechanically prescriptive, and when those themes show up it can totally kill my motivation for a jam. It’s the reason I don’t even bother to tell myself I’m going to participate in Ludum Dare anymore. When I see them, it just feels like I’m making someone else’s game idea. But I’m not claiming that those themes are inherently bad - just that I dislike them. They’re clearly popular, after all: enemies as weapons, 10 seconds, and your life is currency are all themes that have popped up in Ludum Dare, and I personally think they’re absolutely abysmal. It should go without saying that others disagree.
By contrast, one of my favorite Ludum Dare themes was “Growing/two button controls.” As in, you could pick either of both of those. I’m not saying that others didn’t like it (I’m sure many hated it), but I really felt that it left the door open for inspiration. One is ambiguous, while the other is more mechanically prescriptive, so it seems to capture a better range of philosophies behind themes. On top of that, it implies the added challenge to those who want to figure out a clever way of combining them.