A sleuthing Windows Insiders have revealed the name of the next Windows feature update is Spring Creators Update. This is based on the output of a command entered in the latest Insider Preview build from the skip ahead ring. The next version of Windows 10 is codenamed RS4, or Redstone 4, and the version after that is Redstone 5. The spring update includes some new features such as Timeline & Sets (File Explorer with Tabs!), Edge and Cortana improvements, new privacy setting layouts, and People.
Source: https://www.bleepingcomputer.com/news/microsoft/rs5-build-leaks-spring-creators-update-as-the-name-of-next-windows-10-update/