Going against the grain a little here, but I'm quite certain based on many subtle hints, that the Apple Watch case will not change in size/shape significantly at all for at least a few years. It will not get thinner. There's a few reasons:
- The band connector puts a limiting factor on case size and Apple's not going to redesign that connector in such a short period of time as one, or even 2 or 3 years. I expect 5 years or longer with this band connector. It's plenty thin for all normal watch sizes. Only ultra thin watches couldn't work with it and that is simply not going to happen, not until battery life is a week.
- The focus for quite a while will be on improved battery life. Thinning the device won't help that. Apple only makes their devices smaller/thinner when they can maintain or even increase battery life in the equivalent class of product. They have minimum battery duration goals that guide product design.
- This isn't a technology device, it's a luxury fashion product. The focus is going to be on bands. I expect new bands in less than a year, and at a year. I expect there to be a new Apple Watch next year with improvements, but, the same case design. I expect at least one new material/paint job. Probably copper, but, possibly gold aluminum.
- Apple didn't sell this product on tech specs. They sold it on a set of arbitrary capabilities and a ton of fashion/design appeal.
I think this product trajectory (basically staying the same but getting silently better inside over time) matches the Apple TV. Apple is also planning to report income along with Apple TV with no break down. This product is probably one of a handful of coming "iOS extension" targets. I expect a new Apple TV this year with app support but not how people think. I expect the "apps" to come in the form of iOS extension just like the watch apps are going to do. I imagine they include CarPlay in this same group, and I would be surprised if we don't see more similar iOS extension targets in a few years.
That's my take anyway. Minor changes, not thinner/smaller, focus on fashion and maintaining current capabilities while increasing the performance (battery, speed) of those capabilities, keeping compatibility with bands, and generally focusing on the product as it is now for a few years at least.
Apple Watch is an iPhone accessory, not a product platform to be pushed hard towards new revisions.