For migration we recently added a feature to export your user data. But “real” migrating accounts is something I put on our “todo” list, though it probably also first needs a proposal to define how it should work exactly (should it still work when the original instance is down?) As soon as we start giving users more control over their private key issues start appearing like not having any infrastructure for key rotation / revocation. Without that it will only work when the original instance still exists.
I’m not sure if by tagging users you mean linking / mentioning them? Or adding tags to them like you can tag posts / users on other platform. For tagging in general there’s a pending proposal github.com/LemmyNet/rfcs/pull/4 . So far it focuses on post tagging though to reduce the scope. I think the goal is going to be to start with one kind of tagging and add more kinds of tagging later.
For improving cross-instance linking (both communities, posts, and users) we also have a open milestone. There’s a few spitballing issues about it, but no real concrete proposal on how to build it yet.