Ever since the original Apple Watch premiered in 2015, owners have been frustrated by the absence of third-party faces. And it's hard to blame them -- while Wear OS and Pebble devices have had them for years, Apple Watch owners couldn't do much more than customize existing faces. Thankfully, there might be relief on the horizon for individualists. The 9to5Mac team has discovered code in watchOS 4.3.1's NanoTimeKit that hints at future support for third-party faces. It's just a placeholder, but it's explicit: one of the kit's log messages says "this is where the 3rd party face config bundle generation would happen."
With that said, we wouldn't expect the feature to surface in the immediate future. From initial appearances, whatever third-party watch face support exists is preliminary at best. We realistically wouldn't expect to hear about anything more substantial about support until WWDC in June, and that's assuming the feature is coming at all. Still, that there's any mention in the first place is promising to anyone who's grown bored of the limited choices they have today.