Once you master a language, only learning new libraries will not be enough for you to advance your skills further. Sure, some library might come along and challenge the status quo, but generally you will tend to stick with the patterns you're comfortable with. You get your job done just by going through the motions, without thinking much about what you're doing.
That's where you stagnate. The last thing you want to do is fall into complacency.
You want to learn a language that will make you feel dumb at first. You need to be disoriented in order to grow as a developer. This is where you'll push the boundaries of your knowledge into new territories.
My personal suggestion would be Clojure. And that's a story for another article ;)
What would be your suggestion for a programming language that goes off the beaten paths?