Apple can't allow it. Will never happen. Even with the strict guidelines they have on the App Store we have thousands upon thousand with crappy apps. Can you imagine a Watch face store being popular with about 8 approved Watch faces and nothing more? They would either have to allow everything and get sued or only allow what they ship with the Watch already.
If they did find a middle road, who gets paid? I'm sure Rolex would block everything resembling their designs and why would they themselves sell a Watch face when they want you to wear a fine $3000+ jewelry made by them?