Don't want to go aginst the grain here as I dream of (another) digital RF myself, but what makes you think that Nikon/Canon are going to take the valuable resources away from high demand, high turnover DSLR R&D and production and invest them into uber-nieche product like that? Supply and demand my friend. Bring yourself and a couple of hundred thousand friends and we're on. If you think that members of RFF and like forums constitute substantial market for Nikon and Canon you're dilusional. Trust me, I'm in the industry and know the numbers, demand for such cameras is nothing compared to DSLR demand.
And yes, they could produce a very nice (APC or fullframe) compact, but do you realise what the cost would be? The price would cut into lower DSLR or even higher and they would canibalise each other. Not good in business terms.
The only reason Oly and Pana are doing it is because their DSLR market share is not even visible in the big picture, they're just a thin line running invisibly next to zero while Nikon/Canon float up there close to 40% each. Oly and Pana have nothing to loose, it's actually good they diversify because they have no place in DSLR market, business-wise.
Nikon/Canon do not posses unlimited resources, they already struggle to meet DSLR demand in the low/mid range sector. Jumping into another sector would only hurt them in resource department.
Nikon already gave you a couple of nice anniversary "S" models. That was a nice gesture that covered an actual demand and served a symbolic purpose. Unfortunatelly that's as far as it will go for time being.
I'm not saying such cameras are impossible, just not now and not from these two guys. If you realistically want a digital RF-like camera pressue the underdogs - Leica, Oly, Pana, etc.
As far as the niche goes, a high cost digital MF would be a lot more realistic.
Again, I would personally LOVE to see such a camera, but the reality dictates otherwise.