That’s a feature I’d love to see implemented in the future, if you could log in to any federated service with one fediverse account. I think some newcomers might have hoped for this feature when they heard that you can see content from any platform with one account.
Unfortunately, you are currently tied to the UI and capabilities of the service you registered with. The features of a service like Mastodon, Lemmy or Peertube could actually be independent of where the account is stored. I’m thinking of Solid pods and it’s ActivityPub implementation AvtivityPods where an account is like a capsule that can store data of all kinds and from different services.
With a registered account on one instance, you could log in to any other service and consume the content in the intended environment. Or maybe we could decouple the services completely from the instances hosting the pods, so that you have servers that only store the data from all the services and those that provide the front end of the services. What do you think about this?
Once upon a time, we had OpenID. You could host your own identity server and log in to websites with it.
Then the social giants introduced third party SSO buttons and OpenID kinda fell by the wayside.