It would be useful to extend the ChangeRequestSCMHead with some more metadata which plugins and builds can use.
Two notable missing pieces are Labels and Draft status. Both GitHub and GitLab support these concepts on change requests.
Adding these to scm-api initially could be followed up with implementations in github-branch-source and gitlab-branch-source.
branch-api's BranchNameContributor is an existing place this metadata could be put to use, with new environment variables for draft status and a delimited string of applied labels.
Other uses would likely come in the form of new plugins.