- User Answers: A particular user's newly posted answers to questions. Every answer posted by the user gets published. Edits to answers would be published as well. Ideally, it would be nice to differentiate somehow between new answers and edits to old questions.
- User Questions: A particular user's newly posted questions. Edits to questions would be published as well. Ideally, it would be nice to differentiate somehow between new questions and edits to old questions.
- User Favorites: A particular user's newly added favorites. It would be nice if it also included changes to favorite questions in the feed, but identified if it was a newly added favorite or a modified existing favorite.
- User Reputation: Summary of a particular user's reputation changes. This should likely be a once/day publication.
- User Badges: A particular user's newly obtained badges. This should likely be a once/day publication.
For the user feeds, it would be nice to see them extended at the network profile level. What that means is that my network profile would, for example, have an RSS feed for user answers. This feed would include all of my answers to any question on any site on the network.
- Site Badges: Badges awarded to users on a site
- Site Tags: Tags created on a site
Site Tagged Questions: All edits (including old posts with the tag added) to questions with a given tag. This may or may not be covered under the current tag RSS feed.
Network Site Status Changes: We have a feed of new proposals to Area 51, but this may be a superset of that. I'm not sure if proposals should be included, but it would be nice to know when a site enters Private Beta, Public Beta, Graduates, or Closes. I know there's some talk about the graduation process, but this would be a feed of site status changes, regardless of what those are.
I believe that something like the Reddit approach for feeds may be useful for some of these, if privacy is a concern. Perhaps each feed could be associated with a key instead of the user's ID. If the user wants to consume the feeds privately, they can do so. They can also choose to post the link to them and regenerate the code if they choose to hide them again.
I would like to acknowledge that all of this data is indeed available through the API. However, the API is not necessarily friendly for the non-technical audience. As a software engineer myself, I could probably build applications that link up the Stack Exchange API to outside services. However, that may not be feasible for all users - look at sites like Project Management, The Workplace, Cooking, Gaming. Users may not have the skills and knowledge needed to build software to do this.
Instead, we can look at integration services, like IFTTT or Zapier. Although perhaps there could be a Stack Exchange channel provided by these services, that would limit use to these services. As an alternative, both IFTTT and Zapier (and I would suspect similar services) provide a method to consume an RSS feed and do actions on items in it.
An RSS feed would allow me to integrate Stack Exchange to other services - mobile device notifications, email, Twitter, LinkedIn, Facebook, WordPress, Tumblr, Evernote, Trello, Todoist, Google Calendar, and even smart home devices like Hue. This would be something that even non-programmers will be able to accomplish.
In addition, the use of RSS could be considered more open than the API (since it's easier for a wider audience to use). It could help promote the discovery and linking of Stack Exchange content to other sites.
Here are some specific use cases for these feeds:
As a professional in software development, I want to share my posted answers to the software development Stack Exchanges that I participate in with the world by posting them to Twitter and LinkedIn.
As a Stack Exchange user, I want to add my favorites to ToDoist so I can remember to revisit them when I have more time, perhaps to provide an answer or update an existing answer that needs additional information.
As a Stack Exchange user, I would like to get a digest email of badges obtained across the Stack Exchange network.
As a Stack Exchange and Reddit user, when I ask a new question on Stack Overflow with a particular tag (note that IFTTT allows for keyword or simple phrase matching, other applications may offer more specifics), I want to post it on a specified subreddit.
As a Stack Exchange user, I want to get a notification when a site in the network changes status. Example: a new site enters public beta, a site is closed.
I think there are more integrations than this. For example, if you have a Wordpress blog, there are plugins to display RSS content as widgets - SE users with a Wordpress blog could leverage this.
I think there's also the advantage that people can subscribe to feeds and not worry about querying the API, hitting rate limits. Application developers can use any public RSS feeds as an alternative method of accessing a subset of the API and consuming it in various applications.
As an interim solution, I'm going to look at Stack2RSS again. However, it would be nice if there was an official solution provided by Stack Exchange, perhaps deploying something like Stack2RSS internally and updating the feeds at an appropriate rate that may be beyond the limitations of the API. But it could be an interim thing that I can use to generate all of the feeds that I want.