I’ve planned to dive into the authentication side. I think it would be nice to be able to host multiple services in example.com and login on those via 1 login. This way you can post on lemmy and kbin as user@example.com, follow people on mastodon, be reachable on matrix all with the same identity. (And when you want, even receive email)

I doubt it would be useful for open instances, but it could be nice for self hosting.