Yeah I think we should separate these two quite different wants;
1) "I have a Fediverse account, why can't I log in to this new Fediverse service with it?"
and;
2) "How can people subscribe to my [various] Fediverse activity"
My suggestion to #1 is to let (some) Fediverse services also be authentication endpoints for others. That is, the Bookwyrm instance you sign up at will authenticate through social.coop just like we can log in to various services on the Internet with our Google and Facebook accounts. This is known technology, "just" needs implementation and support. At the same time this would allow for pure "id services" so that I could be person@somewhere no matter which underlying instance I'm actually using.
#2 could probably be solved by some aggregation service, but I haven't thought a lot about it. While we talk about Fediverse/ActivityPub as "one protocol" we also need to realize that many services use their own extensions on top of it.