What are you thinking, in terms of both the fourth casing option as well as "unexpected" surprise?
The unexpected surprise with the Apple Watch would be the casing if that _is_ something that would be true. The only real rumor that’s been indicated has been additional health sensors, increased battery life and a larger display, but I’m sure Apple will have something that was not rumored or discussed. Or, Maybe they offer an additional casing color option as well. If they were offering an additional casing, I think it would fall somewhere between aluminum and stainless, perhaps titanium (Obviously a guess). Apple invests a lot into their metals throughout their product line with the MacBook, iPhone, Apple Watch etc.