I'd highly recommend learning a functional language. If you're coming from Rails, Elixir + Phoenix, would be a logical step into the functional world. It's a w

I'd highly recommend learning a functional language. If you're coming from Rails, Elixir + Phoenix, would be a logical step into the functional world. It's a world that-- at least for me-- changed the way I think about programming and improved my OO code along the way. Another useful tool for your toolbelt would be a statically typed language. Go, C, C++, C#, and Java are probably the biggest contenders, but there are arguably more interesting candidates out there (F#, Scala, Kotlin, OCaml, Nim, Haskell). I come from a C# background, but do mostly JS and Ruby these days. I really miss static typing. It's something everyone should try at least once. It's not for everyone, but I feel 10x more productive with a good compiler and IDE.