How about sustainable energy, which will be coming up a lot more with global warming and carbon emission reduction.
Community Helpers We are learning about volunteers that work with nature and wild animals. There are lots of groups that do things like raptor rehabilitation or sea turtle rescue. Try to find groups that are nearby because they will often send a speaker for free to schools.
look for books on theme first? Before you jump into any theme you might want to find out if there are enough titles out there at whatever reading levels you have in your class. I would go to Book Wizard and put a theme idea into the quick search and see what books pop up at what levels. If there are enough titles go for it, if not try another theme. http://bookwizard.scholastic.com