The good news is that according to code discovered on watchOS 4.3.1 by the folks at 9to5Mac, developers might soon get their wish. According to the code, it makes mention of third-party watch faces for the Apple Watch, suggesting that support for it could be coming in the next or future updates.
However as 9to5Mac has pointed out, the feature has yet to be implemented which means that you can’t use it yet. However it is an indication that Apple is at the very least considering support for it, although when exactly it will be released remains to be seen, but if we had to guess, perhaps an announcement at WWDC 2018 alongside watchOS 5 could be a good place to start.
Either way we’ll keep our eyes peeled for more information, but until then maybe don’t get your hopes up just yet just in case this feature doesn’t launch.