Aside from current casing materials of aluminium, stainless steel and 18k gold, we believe 1-3 new versions of Apple Watch featuring new casing materials are likely to go into mass production in 4Q15.
It's unclear what materials Apple is looking at; however, a recent report from the WSJ suggested that Apple is investigating platinum. Other options include titanium or even plastic.
Kuo predicts that Apple will sell about 5-6 million Apple Watch units in the first half of 2015 and about 15-20 million units during 2015; however, new casing materials could increase sales estimates for 4Q15 and 1Q16.
The Apple Watch goes up for pre-order starting on April 10th. More details here.