I think it covers 5. Could be extended a bit (especially if in the future there really exists a group with activity that can be presented as well), but it's awesome and very close! Are you the author?

2 was different. I was thinking about what happens when a user clicks on a feed - it's not very user friendly, especially now that browsers just show the XML file content. If we had a mechanism that wraps that request and gives the user a way to subscribe directly via one of the many available feed reader, that would help a lot.

The idea is not completely novel, https://github.com/superfeedr/subtome goes into the direction but I think it is broken. A website carried by an independent group could be a solution. Maybe even something that could help in case browsers are open for some changes here, but don't want to create the UI part. Or when working on extending a CMS and instead of implementing that UI again, we use that mechanism to wrap the feed. As a web developer making it a website was probably an idea near to me, but there might be better solutions.