The US doesn't have to change the physical suze of its wire, just give the wire a square millimetre name.
If you're not planning on actually changing the physical size of the conductors, why introduce another system at all? I don't quite see what you're getting at.
If the problem is just that non-U.S. countries might not understand AWG, why not just print the metric equivalent alongside, e.g. 14 AWG (2.08 sq. mm) ???
