I think that while #ActivityPub provides possibilities, it also has problems if you plan to roll your own implementation like for your blog.
Then you have to fight against follow bots, spam, etc. by yourself. Very much like it was fought when blogs had open comments, but I don't know how bad it is.
Even if #Mastodon doesn't become super de-centralized it's still an improvement, as it allows bigger players to fight out each other. Lot like with E-mail servers.
But how would you design such a protocol to address those problems?
I don't see an answer to "how" in that :)
I'm kidding, well half kidding.
From my perspective, it's not a problem that can or should be solved at the protocol level. Yeah, I agree that it requires human labor, and I'd put it as a social problem that requires a social, not technical, solution.
Me, I'm often drawn toward solutions that are fully decentralized, empowering end users to shape their experiences, especially by using web of trust type techniques to avoid interactions that they'd rather not get into.
And that probably wouldn't integrate down into the protocol