I had an idea that might be interesting to branchable folks. I originally posted it to the git mailing lists there was some discussion there too.

light-weight pull requests:

Anonymous and ssh/http-identified users should be able to push just using git, within the refs/pull/ namespace, to any non-existent branch name or to a branch they created when previously identified, without forking off a new repository.

Advantages:

Removes the need to look up who to send the pull request notification to since configuring that is up to the project itself.

Removes the annoying scenario of having lots of remotes that have been removed after the corresponding pull request was closed.

Moves git hosting services from primarily a model of forests of forks to one of contributions to well maintained ongoing projects.

Allows users to use their preferred git clients to issue pull requests instead of using web interfaces of popular git hosting services.

Creates a new standard for contributing to repositories on all git repository hosting services.

Contributions from people without an account on those services are possible.

Contributions from people without any git repository hosting of their own are possible.

Contributions from people who don't use or dislike MUAs are possible.

Disadvantages:

Pull request spam, could be mitigated with configuration options.

Extra configuration and complexity on the server side. This is once only and means less complexity on the pull requester side.

Will not work with typical setups where the git/http/ssh user does not have write access to the repositories. A workaround could be some sort of hybrid-repository setup with the new refs and objects in a second repository which would be shared by all pull requesters.