I'm personally OK with the old-school way of one account per community/server. All I really want is forums with (1) a nice clean UI, (2) nice mobile app, and (3) open APIs. Most popular forum software meets only one, or even none of these. Lemmy has all three of these. Federation is maybe nice icing on the cake, but I could take it or leave it personally. Maybe that's denying the whole point of Lemmy, but I don't care.
Client apps will likely end up managing signup and credentials automagically. We already do it for certain/acme.
Bluesky has a global identity system where instance accounts are just links to a DID (basically your private key). If you get banned from an instance you have to change your name but you keep all your posts and likes.