Apple should let users alternate between iOS and macOS on this device. They should also enable touch on the iMac and let users run iOS on that as well.
Big Sur already takes a lot of design cues from iOS - they're prepping the merge. They're just making sure the frog doesn't realize the pot is starting to boil.
It would probably be an opt-in thing for apps, but even without any fancy state replication it'd be just fine.