I think this a really good suggestion. As I’m sure you’ll understand, security is a massive deal for us, so we have a bunch of thinking to do about how we would do this well.
I think it’s as much a UX question as a technical one – for instance, we could have a “places you’re logged in” screen (think “Find My iPhone”), but how do we make sure you feel in control without making it confusing? How do we manage automatically logging you out intelligently without compromising security?
What sort of user interface would you want or expect here, to ensure logging in is both flexible (Want to log in on a friend’s device? How does that work with fingerprints?) as well as secure?