There are lot of packages that can't be hosted on the Build Service.

The idea would be a tool where you can say:

emerge Foo

and it goes to the (preconfigured or default) project, checks out the .spec files (which may or may not have the tar.bz2), download the tarballs if needed, build the package locally. Install it, and put the result in a cache.

BUT.... more important than that... recursively do this to build the required dependencies that are missing from the system.

I am pretty sure this can be done using all the stack we are already using: OBS, osc, build, plus some extra magic and integration work.

May be it could also be designed using some recursive packaging, so a spec file that builds an rpm that contains the sources to build an rpm.