Since Windows 8 has massive amounts of DRM built-in, why not just set a spec for xbox.next branded OEM provided hardware? Microsoft does this with phones and used to do it with tabets. In the end, what is different about xbox verses something like a Windows 8 RT device? Things that might go on the certification might look like this:
* Boots under 10 seconds * Conforms to DX11 * Scores at level X for benchmark Y * No crapware allowed * Is no louder than Z decibles, under stress * Supports controller spec * Has at least one HDMI out
One of the major reasons game companies like developing for consoles is that you know exactly what the hardware's going to be. It makes the game easier to optimize, and easier to test (only one possible config with maybe minor variations like hard drive size).
Not to mention that a year after release, you'll get new systems with significantly better specs than the original, and then you'll get games that won't run as well on the original systems.
Allowing OEMs to produce their own "XBox" systems would remove one of the major advantages of consoles unless they were all required to have the exact same spec. Otherwise, you just turn them into slightly more standardized PCs with all the same problems.