Apple still doesn’t allow developers to create entirely new Apple Watch faces, but with watchOS 7, it has freed them up to make and share very many different options.
As of watchOS 7, you can now buy and download faces for your Apple Watch. It’s not what users and developers have been wanting from the start, though, as it’s still not possible to entirely design every element of a face.
Apple’s limitations mean there’s only so much a third-party company can do. Ultimately, it comes down to offering images as backgrounds to Watch faces, just as you could do yourself from your Photos album.