Maybe it would be cool to improve upon flex/bison/jison and make it easier for everyone to create or modify the programming language of their choice with platform plugins that let people connect their own programming language to the platform of their choice (JVM, .Net, straight up compiled into a binary). I'm really mostly thinking about syntax of the language really, but you could go further and customize the typing…
Clojure is specifically designed to be "hosted" on another programming language. Also, it has facilities that let you create your own control structures and otherwise make it easy to create a DSL. (Domain Specific Language)
This basically gives you: Custom language -> some intermediary form which would probably end up having some restrictions on language design -> to the thing you want it to run on.