I've always wanted to write a planetary simulator. I think clojure might be a decent language for it, as long as you avoid floating point as much as possible and use clojure's Ratios class instead.

  user=> (type (/ 1 3))
  clojure.lang.Ratio
  
  user=> (* (/ 1 3) 3)       
  1

I'd really like to see an arithmetic macro for clojure to allow infix notation, constants, and lazy evaluation. Something like (arith 2m/s^2) would be crazy awesome.