I'd personally look to find what language fills the gap that the current set of languages I know don't fill. Clojure is my filler for lisp language and language with good macros support.

I use Ruby in the slot that many have Python in but with added coolness of metaprogramming not found in most scripting languages.

And to add to that a bit, I found that recently it's been getting harder finding new languages with enough new patterns and paradigms to keep me interested in learning and mastering them.