A job? Then Node is sufficient. Go wouldn't be hard to pick up on top of it to make yourself more attractive. PHP is somehow still a stalwart in this realm, too, and there's lots of demand for C#.NET out there.
Do you have a project in mind? You'll want to consider the particularities of the idea and its requirements to decide if a specific language/runtime/framework could serve you significantly better than one you already kinda know.
Just to learn more? This is the hardest. Learning just to learn is very enervating. But if you've got the spirit, I would suggest trying something more exotic to what you've listed, like Haskell or Clojure, to explore different ways of approaching the problem at a high level, or rolling some socket code in C for a deep dive into the low-level specifics of web backends.