Sounds useful but there is a risk of centralization and it would be fairly easy for the third party to bury hashtags they dont want to propagate. Effectively causing censorship even though the notes are still on the relays. If the relays do it, then it's probably fine, but how would clients blend together all the suggestions when each relay is boosting different priorities?
Discussion
It would be opt in and ideally many options of providers you can pick from within the nostr client as I see it. Sort of like a browser has "pick default search engine" your client has "pick your content discovery algorithm"
Good analogy. I remember #[5] talking about allowing different "For You" algorithms on Twitter in the congressional hearings, but sadly it was never implemented. We actually have the freedom to explore this with #nostr. As more people onboard to nostr, we definitely need a better way to filter and surface relevant content.