Too bad night photography won't work, I would have liked it. Looking back through the archives, we did "Food" in September 2009. As for John's ideas "closeups" or "abstracts" or "double exposures". I recall we tried to do close ups, but I think we could not do it because not everyone had macro lenses. We did do abstracts in January 2009, but the title was "Minimalism". As for "Double Exposures" that would be a new one, but perhaps a bit too challenging for some. I don't know. We could of course repeat "Food" or "Abstract" those themes can be done easily.
An idea idea that I could offer would be "My World" where participants would show images of the environment they live in. These images could be taken inside your homes, around them, they could be landscapes in your area etc. All the ideas offered so far "night photography", "food", "closeups", "abstracts", "double exposures" could be used to show your world. Just an idea. I am sure we can find a theme.