The really intriguing thing about ActivityPub, at least to me, is it's capability and potential to be a bridge for many other protocols.

For example, here's ActivityPub via email: https://apubtest2.srcbeat.com/apas.html

That page also references the longstanding NNTP(Usenet)-email bridge that existed for the linux-kernel mailing list, so we could get ActivityPub to Usenet.

In fact there are a couple of RSS->Mastodon projects out there already, such as https://github.com/dariusk/rss-to-activitypub or https://github.com/jehna/mastofeeder