"Folders," if you really feel you MUST do something.
If you really insist on doing something new, give us the ability to create folders like Gmail does, especially if I want to save important types of emails without them being in my Inbox.
Otherwise I like being able to sort multiple ways. Don't EVER get rid of that.
63rd
ranked

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Hey Roger, not sure we're following. You can create folders on Yahoo Mail, the Desktop version. You can expand the right hand column by hovering your mouse at the edge, waiting for the double arrows, from there you can expand folders. The three dots by hovering over the folders will appear and from there you can create new folders.