If you've been following the world of Microsoft for the last few years, you'll have noticed the company's design language change quite a bit. Windows Phone 7 Series introduced the Metro design; it was brought to the PC with Windows 8, and it has evolved from there into what we see in Windows 10.
According to a new report from Windows Central, the next stage in that evolution is codenamed Project NEON. The new project is not meant to be a complete overhaul of the current design, but rather the next iteration of it.
One thing that NEON aims to accomplish is to straighten up various inconsistencies that can appear throughout Windows 10. UWP developers can currently create "their own context menus, app bars, hamburger menu designs and more", which can end up in a fragmented interface. NEON would offer stricter guidelines.
According to the report, Project NEON has been in development over at Microsoft for over a year, and should appear in Redstone 3, which will arrive for the general public in late 2017. Of course, Insiders will see it sooner; the Creators Update is currently expected in March, so we'll see Redstone 3 Previews with real changes a few months later.
Update: According to additional information, it appears that NEON may be more concerned with app user interfaces when transitioning between the desktop environment and the physical environment via holographic or AR (Augmented Reality) experiences. It's also believed that NEON may leverage other effects including textures, 3D models, and lighting, amongst others.