I always thought Microsoft should've used Rosetta/Quicktransit software.
It's what Apple used to run PowerPC apps on Intel Macs. Apple just licensed it too so MS could have as well. The PowerMac G5 also used the same exact architecture that the Xbox 360 did (both based on the PPC 970).
The only thing I am not sure of is if the One's CPU is strong enough to handle PowerPC games. From what I read, Rosetta was great at emulating things like Office and Photoshop but had problems with programs like Final Cut Pro and games (both of them lol), but I never got a confirmation (I went from an iMac G4 to iMac Intel).
Either that, or do something like what Sega did with the Genesis being backwards compatible. Basically have the CPU sold separately then you just connect it to the I/O in the back and the One emulates the 360 GPU and you get native B.C. Microsoft could then sell it for like $49-$59.
