Unfortunately we do not plan to support further improvements for RSS due to possible privacy issues.
The problem is that every collection have public and private RSS feed. Private have uniq cryptic url, but if you share it somewhere you will open full read access to your collection.
So we suggest to use our API or upcoming IFTTT integration instead.
IFTTT integration will have ability to filter your bookmarks by collection, tags, type and other fields. By connecting it with Feedly (or other RSS reader) you will have super private, reliable and flexible solution.