I’m willing to bet that you could attach an iPad Pro to a MacBook Pro, where the screen would normally go. You could un-dock the iPad and use it as an iPad, or dock it and it becomes the Retina Display for the MacBook Pro.
It would be 1 device, made of 2 devices, running 2 operating systems, and syncing files via iCloud or Bluetooth/Wi-Fi. What you’re doing could be sustained from device to device upon dock/undock via Continuity’s Handoff & Universal Clipboard. PowerNap and other background services could keep both devices in constant sync.
This would require “Super-Universal” apps, that contain both Intel/macOS and ARM/iOS versions, but it might be doable.
What are the advantages? It truly means no compromises. iOS+iPad for some tasks, macOS+MacBook for other tasks. Power when & where you need it. Maybe those devices could delegate tasks to one another for off-board processing. The MacBook wouldn’t need to contain speakers, because the iPad would contain the speakers. This would leave more room in the MacBook for additional battery.
Maybe you could mix & match the individual pieces: iPad Air & MacBook Air iPad Pro & MacBook Pro
How would they market it? Maybe call it a MacBook Plus? or dare I say it… an iBook?
It would also help increase iPad sales. It’s a natural coupling, too, since the iPad and Mac sales cycles seem to be similar, which is to say, much less frequent than iPhone upgrade cycles.
It’s just a thought… but I think it’s an interesting one.