It'd be awesome to get extra info on a sender when reading an email. Did this person buy from us before? Is there an open case? An unpaid bill? That data would have to come from outside Missive so this would be more for developers to create their own added context using a Missive API that passes details on the sender and the rest is custom code displayed in an optional sidebar (or pane when hovering the sender address, is that better?).
A bit like this: https://dev.enchant.com/sidebar-app.