Contrary to the other posters, I think this may actually happen: a little research will show that the xbox 360 OS is based on Xbox OS. Some people say this is a fork of Windows 2000, some people say it is from scratch.

If it's true it's based on Win2K, given the current Windows code base is still very portable - we knows x64 and MIPS are maintained already - it makes sense to have a unified code base and update the NextBox.