It seems Microsoft is already thinking about the next-next generation of Windows, as new rumours have surfaced claiming that on the 9th of August, Microsoft began development on the next version of Windows, Windows 9. According to the source, it's also codenamed Windows Blue.
It's currently unclear whether this information is true, but the source seems to claim that Microsoft began development on Windows 9 on August 9. Details are still pretty scarce at this time, but they do have a codename.
Windows Blue is the suspected codename for Windows 9, it's currently unclear why or even if it's true. As stated above, this is a rumour, and could be false.