Erlang/Elixir would be a good bet. Erlang very much has a "model" to it that is hand tailored to a specific niche (reliability), and fits very well in networking scenarios. Adding to that, both languages are gaining adoption and will definitely see an uptick in job postings in the coming years.

If you want to learn a language for the express purpose of expanding your mind, try a postfix language like Forth or Joy. Those are different enough from what you've done so far that it'll be pretty strange to think in that manner at first. I haven't pursued either Forth or Joy beyond messing around with a couple toy examples, but they're very interesting to think about.