Since the launch of Metro in Windows 8, Microsoft has iterated on its "Microsoft Design Language" a few times, with MDL2 being the most recent version to grace Windows 10. According to fresh rumor, it looks like a third iteration is en route for Windows 10, and it's called "Project NEON".
What NEON ultimately is likely to become is "Metro 2", the overarching design of Windows and its apps (and the Windows Phone UI) that adheres to very strict guidelines. It's modern, clean and attractive.
Microsoft's Metro design language was first introduced with Windows 8
While we're still a ways out from "Metro 2" making its debut, it's rumored to iterate on Metro rather than dramatically overhaul it. Some sources say that new transitions and animations could be introduced. Basically, it should become even more beautiful and eye-catching. While aesthetics are not the only important aspect of computing, having a fluid and slick interface can add significantly to overall experience and feel.
For developers, one important aspect of Project NEON is that it won't require much of a learning curve - what you already know isn't going to go change much or go to waste. Furthermore, Microsoft is apparently trying to get rid of a lot of inconsistencies between developers in MDL2, offering more definitive guidelines on app design, which should improve the experience across multiple device types.
Project NEON is estimated to arrive next fall with the Redstone 3 launch. However, initial preview builds of Redstone 3 could appear much earlier than that. Even at its launch, Project NEON might not be fully realized, so Microsoft could continue iterating leading right up to Redstone 4.
One thing that might not stand out too much is the fact that this is all based around Windows 10. These kinds of changes themselves could help warrant a major OS upgrade, but Microsoft is continuing to focus entirely on its latest OS. This approach so far lives up to the idea that Windows 10 will be the "last monolithic" Windows release.