A future version of Apple watchOS is likely to bring a major change: the ability for third-party developers to create custom watch faces. As of now, this is not possible, but code found on watchOS 4.3.1 indicates this feature is in the works.
Specifically, the code currently just includes a log message that states "This is where the 3rd party face config bundle generation would happen."
So the implementation isn't currently there, but the message itself is enough to suggest the functionality is incoming. Of course, Apple may drop the plan altogether if things don't work out as planned, but let's hope that doesn't happen.