Many possible solutions have been proposed, but the trade off is complexity and number of events to download. Better relay selections would largely fix this without any re-designing. Here's a new idea though: a snapshot event which contains any other event in the content, which clients can publish if they're not sure they have the latest version.
Discussion
Consider combining individual follow/unfollow events with periodic snapshots that include all events at specific points in time, featuring pagination with a continuation event ID (a pointer to the next part of the snapshot).