I like this idea and the use cases that were proposed so far! I know this is an older thread, but the conversation was raised again here by @friedger as a focus for supporting the Stacks community.
Regarding W3C standards, if not ActivityPub maybe WebMention is a better fit? I just came across it in reading about the IndieWeb and some of their publishing recommendations. The proposal is from 2017 but there might be some things we can learn from their structure at least. Some more info linked below:
- WebMention W3C Spec
- WebMention Test Suite
- WebMention on the IndieWeb wiki
- WebMention.io as a service to easily receive webmentions on any web page
- Brid.gy as a webmention/backfeed service from social media to a website