The ability to be able to clearly see all email activity (opened, clicked etc) next to the mail, keeping this information sperate/viewable from the main inbox is extremely useful.
This would be amazing!
This already exists – it's literally also called Activity and is visible in the pinned section in the bottom left underneath "pins".
@Alex Panagis: He means per-email. Clicking an email would show the activity for that specific email, off to the side like where "latest conversations" etc.
@Tristan Lawrence: Ah ok – yeah I can see that being quite useful! :)