It depends on what you want.
I learned Haskell some years ago and even though I never used it for professional development, it paid off in the sense that it allowed me to reason about my code in a completely different way. I became a better programmer because of that, no matter the language in use.
There are a bunch of cool languages to choose from: Elixir, Haskell, OCaml, Rust, Crystal, Pony, Kotlin, etc. Pick the poison you get more interested in and go for it.