I would like very much if there were a way to open all unread entries (in either a feed or a folder) in background tabs, and mark them as read at the same time. It seems most natural to me for this to be a menu item on the feed or folder, near the “mark as read” item.
A potential issue would be that if there were a lot of unread entries, this could open enough tabs to render the browser unresponsive or worse, so it might be wise to put a limit on the number of tabs that can be opened in one click; 10 or 20 sounds like a reasonable number, perhaps counting from whichever direction is configured to be the “top”…