A massive leak of Microsoft's plans for the future of Windows 10 Design has surfaced a short while ago, but thanks to Windows Central's podcast, more details have been revealed.
The Microsoft Design Language (MDL), also known as Metro was first used back in 2002, in the Windows XP Media Centre Edition. It has evolved a lot since that and its current implementation is called MDL2. It is used in Windows 10, Outlook, Xbox, Office and basically every piece of software that Microsoft still develops. The successor, or refinement, to this current language, is currently called NEON.
NEON will not be a huge UI/UX change, and will simply be a continuation of what the Microsoft Design Language 2 or MDL2 is right now which means that Redstone 3 will still look familiar but it will be much more polished and feature a more streamlined version of what we currently have.
The recently released Paint 3D preview is a sneak peek of what's to come, as it isn't really using NEON but some parts of it are definitely there. The app has been criticized for not following Microsoft's already lacking guidelines, but it seems like it actually has a reason behind it.
Another thing that NEON will focus on is depth, but unlike Google that focused on shadows to mark depth, Microsoft is planning to give some UI elements real 3D depth, where they will be able to render outside the window. It's still unknown how these UI's would be handled in a 2D environment.
With NEON focusing on bringing Windows 10 Mobile/Desktop/Console and Windows 10 Holographic together, having these 3D elements will be highly handy in an augmented or virtual world you can see through a HoloLens or a VR headset, and if Microsoft's handles this well, possibly on desktop too.