A "face store" would be a cool idea, lol.
I was at the Apple store yesterday and asked about this. I was told by one of the store employees that they hear this ALL of the time from Apple Watch owners with people asking about dial/face options like Android smart watches have.. The 2 store employees I was talking to felt very strongly that 3rd party face / dial apps will be coming in the future due to the demand they experience. Of course this was just their opinion and basically not an official statement, just us talking casually in the store.
One of the good points we chatted about (which I tried to make above) is the big difference between the Apple Watch and the iphone is that everyone has a cell phone. It has become a basic/standard part of everyone's lives. The Apple Watch is an accessory. It is an "add on" that no one really needs. Far different than a device that is a mobile phone. It IS made ot be customized with different straps, bracelets, etc. For that reason there is more chance of seeing customization.
Again, just a discussion and no actual facts (of course.)