I think a purely functional language would teach you quite a lot, it certainly did me. F# runs on the .net run time and you should be able to easily find/figure

I think a purely functional language would teach you quite a lot, it certainly did me. F# runs on the .net run time and you should be able to easily find/figure out the .net libs since you have used C# and VB a good bit. Clojure and ClojureScript are really great options. Clojure is a lisp that runs on the jvm, plenty of great web frameworks to work with and a metric ton of libs available to the jvm. Clojure Script is Clojure targeting a JavaScript runtime, it works great with node.