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 system and how scopes work and go all the way down the rabbit hole as much as you want. As long as the underlying base system is there, and it runs on the things you want it to run on, it would be fine.

Basically: 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.