Slightly off topic, but why do so many people think the next watch will be thinner? Logic dictates that the thickness will stay the same because there are plenty of watches (mechanical and smart) that are of similar thickness to the current Apple Watch.
The watch is not particularly heavy (an argument for making computing devices thinner, and why the iPhone has slimmed down so much over the years).
It seems Apple would be far more likely to keep the same case dimensions for at least one more generation and focus on the electronic hardware and improve it. All while adding more case materials, bands, etc.
People will be less likely to buy a set of bands with every watch if those bands cannot be used with the next generation.
Apple knows this. These bands cost more than iPhone cases.