What we’re expecting
Bloomberg's Gurman says a major design shift is unlikely for Series 12. The Series 10 introduced the current thinner, larger-display design in 2024, and Apple typically holds a form factor for four or more cycles. New colours are expected, since Apple refreshes its palette annually, but case dimensions and silhouette carry over from Series 11.
Apple Watch Series 12 is shaping up to be a new color, same watch. The rounded square case introduced with Series 10 in 2024 isn't going anywhere. Apple typically runs a design for four or more cycles, so Series 12 holding the same silhouette is exactly what history would predict. New shades will show up, because Apple refreshes its palette every year, but if you're waiting for a redesign to upgrade, you're waiting at least another year.