Funny, just a while ago was pondering on what a "github for hackers" would look like.
My personal answer would be: it should be a hackable webapp, where users can implement or tweak features at will.
The ingredients being the usual:
- a plugin system
- a function hooks system
- many, many available options that are never taken away (i.e. breaking API changes).
When was the last time you used a webapp even remotely like that?