You can already do that with an iPad (sans fat OS). If you're using Blink Shell (https://blink.sh) the external display is independent of what's on the iPad too, which works really neatly. This is the exact setup I used as my main dev machine in a previous role.

Would be very nice to see if this works on the new iPhones. A thin client with decent security in your pocket with keyboard/mouse/display at both home and work seems like a very approachable computing setup.

Photo for reference: https://twitter.com/_______kim/status/1348736952330301440