I’d like to add one more project idea to the list
6 - Automatic git cache maintenance on the controller
Recent versions of command line git (since 2.30.0) have a set of commands for “git maintenance” that can be used to regularly update and maintain the contents of a git repository. I think that a Google Summer of Code participant could review the features available from “git maintenance” and identify ways that the Jenkins controller could use those features to manage the cached copies of git repositories.