Making macOS- and iPadOS functionality co-exist on existing devices would, IMO, result in a spaghetti code nightmare, with all that entails for the user, i.e. bugs and general weirdness.
Thinking outside of the box, to do it right, you'd have to go back to first principals. By this I mean initiating a completely new design project. I'm talking about both hardware, firmware and a new OS to run on it. This would also require its own fully understood project mission statement.
Providing compatibility with existing apps, both OEM & 3rd party, would be, um, "a challenge".
Such a project would require so much focus and energy that, once the marketing- and financial dept heads (not to mention the shareholders) heard about it, whoever is CEO at that point would need to be a mighty charismatic visionary to be able to overcome the internal inertia required to even get the show on the road.
I'm not saying it can't happen. But it's not in the category of "hey, it's just a quick mod"!