What about tag wikis and excerpts? They are also (usually) contributed by users, and can be revised. However, at present there are no generally-accessible timelines for tag wikis (e.g., https://meta.stackexchange.com/posts/168600/timeline) or excerpts (e.g., https://meta.stackexchange.com/posts/168601/timeline). (There might be one visible to moderators, perhaps.) (At least, not on /posts/<id>/timeline.)
(I had a bit about making the timeline accessible, but turns out, that was status-completed this year: Add a link to the timeline of a post)
