Take full advantage of living in the 21st Century and put the Web to work for you. Pose a question on social media, create an online poll, ask the readers of your blog, or organize a giveaway.
Find a way to inform people that you’re looking for a central theme for your next project, and let your peers take it from there. If you’re lucky, you will end up not just with ideas, but also with one or more volunteers/helpers/partners for your project. Do thank all the people who pitch in to solve your creative dilemmas.