Microsoft have a new tool in beta for creating immersive prototypes for virtual reality called Maquette. The tool runs in VR with Windows Mixed Reality, Oculus Rift and HTC Vive.
It seems the idea behind the tool is for you to create immersive environments for VR projects from the virtual reality, rather than trying to design a 3D world in a 2D workspace. You use the VR controllers to place and manipulate objects in the 3D work. The app was originally an internal tool for Microsoft designers to use to created mixed reality.
Looking at the documentation it seems you can export the project to FBX or GLTF and then bring the project into Unity so you can use it to create full games or apps. You could also import the FBX file into Windows Mixed Reality and use it as your home environment.
Thanks to WinObs for the heads up!