John Gruber, Daring Fireball, on custom faces for Apple Watch:
I just can’t see Apple ever allowing these sort of watch faces for Apple Watch — that’ll be left for the jailbreak crowd. A few weeks ago I thought third-party watch faces would be like third-party apps were for the iPhone — something that wasn’t there at the launch, but which came sooner rather than later. Having spent three weeks with Apple Watch, I feel differently now. Apps are the apps of Apple Watch — that’s where there will be thousands of third-party designs. Watch Faces are different. They’re more fundamental to the device.
Who knows. But I do think that - for now - any custom faces would be a result of partnering or licensing deals - like the current Mickey Mouse face, as John points out.