It's hard to say without knowing what's already in your current toolbox. But a partial list might be:
* Erlang - Novel approaches to fault tolerant system design. Fail fast and recover somewhere else. Functional without the Type wizardry so you learn about immutability, pattern matching, and the typical functional language tools like folds, maps and so on.
* Clojure - A lisp that runs on the JVM. Learn about Code as data and how that empowers meta-programming. Also a functional language with interesting takes on how to share data in an immutable by default world. Lazy collections of a sort so you can learn about laziness.
* Haskell - Don't let the Category theory stuff scare you. Very interesting type system that can open your eyes to new ways to abstract things. Speaking of abstraction Haskell takes abstraction to a whole other level. Sometimes you can't see the program for all the abstraction going on. Militantly functional.
* Forth - Stack based programming, very small and compact. Radically different than anything else.
* Rust - Maybe you want to go a little bit closer to the metal for a change? Forces you to think about exactly how you share data in a multithreaded world. Targets the same world as C++ but perhaps with an easier to enter community.
* Python - Cleaner more elegant language in the same family as PHP with a very welcoming community. Batteries included so it's easier to get started than any of the others.
* Go - Concurrency done right. Lots of tooling builtin to the distribution toolchain.