I’ve done something like this, with RSS feeds. Read !meta@rss.ponder.cat to see the existing communities, and how to add a feed to an existing community.

The concern about spam is real. A lot of these exist, for example one for Hacker News and a whole instance for Reddit, and a lot of people including myself don’t like those. I agree with you that it’s a good idea but it’s necessary to be careful that it remains a useful seed of content and not an overwhelming spew.