MLS would appear to be a pretty fundamental building block there. MLS can be a bridge between Nostr, atproto, Farcaster and others, and MLS can really scale. Example of one of the many ongoing atproto MLS discussions:

GitHub

Proposal for AT Protocol Instant Messenger · blacksky-algorithms/rsky · Discussion #70

Atproto Instant Messenger (AtprotoIM) Architecture Overview Overview This document outlines the architectural decisions, cryptographic consideratio...