Well, what is this hidden in the beta version of watchOS 4.3.1? As noted by 9to5Mac, it is a reference to a "package of third-party face configurations," which sounds like Apple is about to open the device to developers who want to make their own watches.
Unlike operating system Wear (for example, Android Wear), watchOS has never been compatible with third-party displays, since the first Apple Watch came out in 2015. One of the benefits of having a smart watch is that it can change the face whenever you want, but until now your only options in the Apple Watch have been made by Apple.
It's true that some of those watch faces are very nice. But allowing other developers to participate in the event would undoubtedly see an explosion in the number of available faces: almost all tastes and styles would eventually be taken care of. Probably.
Nothing official yet
This is obviously just a bit of code and nothing else for the moment. Apple has not said anything about the change, nor when it will be released, and probably will not for a while: the WWDC developer conference in June is a good bet for an announcement about the next batch of watchOS updates.
With Google renaming its own portable platform and cheaper alternatives like Fitbit Versa coming onto the market, Apple must feel the pressure to stay on top of its game in the wearables department. The exclusive applications of Apple Watch are quite thin, so they could help the device to shoot in the arm.
Of course, as the operating system attrition is shown, allowing third-party developers to create their own covers would lead to a lot of regular designs, as well as many attractive features, but presumably Apple would launch some kind of system detection, as it does with full applications.