There is code in the latest version of watchOS indicating that the Apple Watch may be running third-party watchfaces in the future. That is currently not possible. It is unknown if and when the function would come.
The code can be found in watchOS 4.3.1, reports 9to5Mac . One of the files in the NanoTimeKit service seems intended for communication with Xcode, the program for macOS where developers can create and test apps. In addition, there is a message in the file that there is the place where developer code for watchfaces would appear.
At present, the Apple Watch only has watchfaces that Apple has made. However, developers can make complications for Apple’s watchfaces, so that information from apps can get into the watchface. Other operating systems and smartwatches, such as Wear OS from Google and Tizen from Samsung, do have support for watchfaces from third-party developers.
Within two months Apple will organize its developers conference WWDC. That is the place where the manufacturer often shows great new functions for his operating system. These will then be upgraded in the autumn.