Looking for a unique birthday party theme for son's 4th birthday.?

My son was born in January, I know I am planning ahead though. We are pretty sure that we are going to have his party at a party facility (bounce houses and ect..) I want a different theme for him this year...we have done Noah's Ark, Elmo, and Trains. I don't want licensed characters. His interest is trains, but we did that for his 3rd birthday. I appreciate any suggestions. Thank you!