Cultural clothing. Pick a country/culture for the people to draw. I personally favor Japanese and also believe it to be the best suited as a drawing theme, especially since virtually everyone on DA loves anime and Japanese art.

Or, have them draw a certain culture's clothing based on a specific season. For example, the Japanese wear brighter colored and light kimonos during the summer and darker colors and a heavier kimono for the winter. The season also influences the patterns on their kimonos.

I know a lot of these people think that Halloween is a good idea because of October, but I personally think it's such an obvious and easy choice.