It would seem to me that some variety of Lisp would be the ideal candidate as a sort of runs-everywhere language, a thin portable base language that runs on top of different runtimes, offering easy integration with whichever it is running on.
Basically, something like a minimalist Clojure but not just for Java. It would be able to run atop the CLR, JavaScript or the Objective-C runtime as well. The interface with the host platform may be different, as long as the core language works everywhere. Ideally the core would be tiny.
Though Forth is only really elegant, when implemented in assembly. Forth requires to generate code on the fly. For something like the JVM, this means to implement your own bytecode interpreter or hijack the class loader. Neither solution comes close to the simplicity of writing some machine code into memory and jump into it.
Basically, something like a minimalist Clojure but not just for Java. It would be able to run atop the CLR, JavaScript or the Objective-C runtime as well. The interface with the host platform may be different, as long as the core language works everywhere. Ideally the core would be tiny.