It seems Apple is preparing to open up its watchOS platform to developers even further, with news that code found within watchOS 4.3.1 indicates that third-party watch faces will likley be supported in future builds.

A portion of the code from the latest watchOS build contains some placeholder text which states "This is where the 3rd party face config bundle generation would happen", indicating that that part of the code is where third party faces will effectively "slot in" when downloaded by a user. This information was reported by 9To5Mac, complete with a screenshot of the code.

The functionality is not currently supported, but this bit of text would suggest that something is being planned for a future version, if only tentatively. It's true that as this is such a work-in-progress that it may go no further and the implementation could be dropped if Apple changes its plans. However, opening up the faces feature to third parties does make a lot of sense when it comes to adding to the appeal of the Apple Watch to consumers.