There's already lots of great solutions for RSS. Ditto for torrents. You can't build a wysiwyg editor that will work across the entire web, there's countless formats which are specific to each website.
They just need to focus on building a top-notch web browser, no unnecessary bells or whistles.
If your goal is to let people self publish something like Twitter/Facebook/Substack posts with embedded video, you don't need much more than a basic rich text editor with some CSS themes (and ability to edit/share custom CSS for advanced users). No JavaScript needed (it's not like those sites let you use it).
Torrenting should have built in support. Torrent support is needed to be able to use magnet links as a src for media. RSS is less necessary, but built in support would be one component of a decent home page like iGoogle tried to build back in the day.
When i try to consider what that really means, it leads me to think it actually encompasses a large and ever-increasing scope of features that have just become the norm. Browsers are becoming almost a general purpose OS.
Also, some of those features clearly require network effect to function which entails having it officially supported and suggested to the users (or even making it default).