Opening up the watch face completely for third partys would be a PITA (for Apple) because in no time there would be tons of apps with copies of watch faces that are other watchmakers' intellectual property. As opposed to Google and the Android hardware makers, Apple cares about stuff like this. Also, all Apple Watch faces are bespoke and high-class. The only times you see an Apple Watch with an ugly watch face, it's the "photo" watch face with some hideous picture the user chose him-/herself. But yeah, it would be nice if Apple offered an rectangular analog watch face beyond the Hermes ones. I can imagine that we'll get a strictly curated marketplace for watch faces sometime in the future.