According to fresh rumors, Microsoft has switched to kernel version 6.3 in the upcoming update to Windows. Windows 8's successor's screenshot has been leaked to the public by the famous underground WZor team:
It is not clear if this image is real or Photoshopped. I can't see any reason to change kernel version number, because it might break compatibility of existing apps. However, it seems to be real and this means that some major updates are planned for Windows vNext.
Note that the build tag is FBL_ meaning Feature Build Lab.
What is Feature Build Lab
After the reset of Longhorn, which became Windows Vista, Microsoft introduced virtual build labs (VBL) which were replaced by Feature Build Labs (FLB) in Windows 7. Every team working on Windows has its own Feature Build Lab (e.g. fbl_shell, fbl_multimedia, fbl_powershell, fbl_wdk, fbl_tools, fbl_dev, etc. — there are plenty of fbl’s).
Well, it's obvious that we will see a new update to Windows soon. You should not expect many improvements on the Desktop side, because Microsoft is focused on Metro/Modern apps and touch screens. The next update to Windows, Windows 8.1, codenamed "Blue", is expected to contain improvements mostly to existing Modern apps, minor tweaks to the user experience and cloud services.
Time will tell, so consider this a rumor.
Feel free to share with us what you want from the upcoming version of Windows. Are you expecting some new features or do you want just improvements to existing features?