There were rumors of a Winux project at Microsoft that put the Windows GUI on top of Xenix. I don't think it was true. But the idea of moving the OS to a Unix OS goes back to MacOSX. Microsoft could do worse than putting Windows on top of Linux.
Start a new OS, along side Windows, based on Linux/Unix.
Make it as stable as Windows for the every-day user, because let's be realistic, even Ubuntu can't manage it.
Ship all new laptops/PCs with this new OS.
Continue supporting Windows/Server for the enterprise.
That would be so awesome.