Preview at WWDC likely in June, followed by September launch.
Apple Seeds Second macOS Sierra 10.12.1 Beta to Developers and Public Beta Testers
macOS Sierra 10.12.1 beta 2, build 16B2333a, can be downloaded from the Apple Developer Center (developers only), Apple's beta testing website (beta testers only) and through the Software update mechanism in the Mac App Store for those who previously installed the first 10.12.1 beta.
macOS Sierra 10.12.1 appears to focus on bug fixes and under-the-hood performance improvements to address issues that have been found since the operating system's release. Few outward-facing changes were discovered in the first beta, but it does include Photos support for the new iPhone 7 Plus Portrait feature introduced with the iOS 10.1 beta.
Any new features that are discovered in the second 10.12.1 beta will be listed below.
macOS Sierra is a significant update that brings features like Siri support, a new storage optimization option, cross device copy paste, auto unlocking with the Apple Watch, and more.