I think it would be cool to have OAuth server functionality built-in by default. I currently run a multi-site network and am working on having it so that people can login to a new Phabricator instance using their account on the ClassicPress multi-site network.
The problem: While I've been able to get the job done, many other features that would be useful are usually put behind a paywall, like with WP-OAuth. Additionally, I've been struggling to figure out a way to grab user's avatars from the network so that it can be their profile picture for the Phabricator instance as well.
So if there was an OAuth Server core plugin built into ClassicPress by default, I think it would be really great, especially for developers who want to do this kind of stuff like I do.