ML, Haskell, Idris, or Prolog.<p>Because these languages will challenge your problem solving abilities and should improve your overall skill in writing code in any language.<p>If market feasibility is really your thing, then Clojure or Scala for similar reasons.<p>It look some deep investigation in Haskell for me to get fairly dangerous with Scala. (There are weird type system gotchas in Scala that can seem insurmountable at first which Haskell does a better job of encapsulating.)