Current best candidate: Clojure+Clojurescript. From what I can tell, the differences are so small that they can almost be considered the same language (is this correct?). Which gives me the same benefit that JS has: no mental context-switch between client-side and server-side code.
Using Clojure(script) I can do a lot of stuff:
* Command line applications, think everything from simple cron jobs to daemons to command-line utilities (replaces: clunky PHP scripts)
* Backend servers (replaces: PHP, NodeJs)
Potential bonus uses:
* Mobile apps, using either Clojure proper or Clojurescript+Phonegap (replaces: clunky Phonegap apps). I don't know how feasible this is in real life, though.
* Desktop applications (replaces: nothing that I've ever tried). Again, don't know how feasible this really is.
Language features that I like:
* Functional programming
* Good concurrency support
* Homo-iconicity, macros
* Lexical scoping, closures
* Good REPL
* Elegant (AFAICT), not a pile of weird hacks, quirks and gotchas like JS and PHP
* Compiles to JVM (Clojure) or JS (Clojurescript) which makes it/them very portable and able to interact with a huge array of existing code/libraries
So, Clojure/Clojurescript would be my current best bet for a really powerful all-purpose language. It would also be a welcome challenge: I've been doing a lot of functional stuff in the past years, but Clojure would really up the ante there.
Feel free to chime in if you have a suggestion of a different language that could also fit the bill :)