This year is labeled the "Year of Collaborative Music" by MTNA, so you could have just a general ensemble theme already pre-packaged.
One of my favorite themes to do with my students was "World Music." It opened up so many doors to talk about different kinds of music and how each of their respective sounds are accomplished. I compiled a long list of world music repertoire while I was looking for music for this one. So, if you want it, let me know.