This is something I've been musing about for a couple of years now... ever since I wondered what the next-gen consoles might be like, given their convergence with PCs.
What I would really like to see is the reverse of the title - for Windows 8 to run XBox 720. I'd like to see the 720 become a software platform, which could run on both dedicated console hardware and a more traditional PC. The software platform could define a very strict minimum-hardware specification, which would set a consistent base-line for everyone.
And by minimum-hardware specification, I'm thinking much like the strategy with Windows Phone 7. Whilst there is a choice in handsets from different manufacturers, there are some very strict minimum-requirements. Manufacturers are allowed to go above and beyond these requirements, but at least there is a consistent base-line, leading to a very well done development experience (contrast with the 'fragmentation' of Android, or the current Windows development experience, where you have no idea what to expect, except for maybe the little bit of expectation you have from hardware supporting a given version of DirectX).
So Microsoft could continue to produce their own dedicated hardware units that meet the minimum spec, but anyone with a PC that meets the 720 spec could also play the same games and use the same peripherals.
Of course PC hardware would continue to improve with time, but the 720 minimum requirements will be static, so a game should run the same on all Xbox 720 hardware implementations.
(Potentially, game developers could be allowed to make use of the better hardware when it is available to improve graphics, but only as an optional extra where available, and only if the game will still run on the minimum 720 spec. This could be dangerous though if things became too inconsistent, from the gamer's point of view).
So what I'm saying is that games would be certified to run on the 720 platform, regardless of what form that platform takes (so long as it meets the requirements).
Then maybe 2 or 3 years down the line (whenever the next-next-gen 'Xbox 1080' comes around), all that would need to happen is for a new minimum-hardware requirement be defined, with a new Xbox 1080 certification for games. Backwards compatibility with the 720 would be trivial.
Of course, this is just my idea. As good an idea as I think it is, maybe I'm missing something