Just a thought about replaceable events. Overwrites of metadata or follower lists by clients who don't include the data from previous events causes users headaches.

What if a new replaceable event includes the current event's id  as a tag? It proves the publishing client was at least aware of the previous event.

Client could choose to ignore the latest replaceable event if the tag doesn't exist, and fall back to either its own cached values or the last replaceable event with that has this tag. #nip01