I'm mainly talking about unmanaged apps, not apps distributed through the Mac App store. I don't think that x86 emulation is an option, that would be terribly slow.
Ah, Binary Translation at the silicon level. That would be very interesting. Apparently NVIDIA's Project Denver was supposed to do this for x86 but they were unable to get the proper licenses, so they shifted it to do it for ARM. https://en.wikipedia.org/wiki/Project_Denver