The pluggable feed algorithms that users can choose is a good idea from Bluesky. I think it could be done as a relay-like service, instead of it providing the events, it could provide lists of event id’s based on what algorithm the user chooses. Since it’s scoped to the user, perhaps you’d need authenticated relay requests…. We could do it in the client, but then every client developer needs to build it themselves.
Discussion
this idea from #[3] is brilliant 
nostr:npub1wmr34t36fy03m8hvgl96zl3znndyzyaqhwmwdtshwmtkg03fetaqhjg240 not a nostr user here.
With just chronological timelines I'm forced to a massive use of notifications, because I can be possibly be interested in a person and never find him in my tl (different geographical area, few messages, different posting hours..)
My idea of feed algorithm is something that help me as a user to make proper decision if I want to keep following someone.
So bigger evidence or automate notifications for, say, the 10 followers/following I interacted the less in a time period.
Maybe I will discover that, after all, I'm not interested on their content anymore and adjust my feed in time