I'm guessing they'll probably keep the Series 7's design for the Series 8 and 9 just like how they've kept the same design for three generations (Series 0/1,2,3 and Series 4,5,6). That would also give Apple the opportunity to introduce a brand new design for the 10th generation Apple Watch (Series X??) and we might even see the squared edges design then.
The S7 design is essentially the same as S4-S6.
I can't see them keeping this form factor for another 3 generations.
Perhaps the SE stays the same, with an S8 and S8 'rugged' being squared edged. The rugged version going larger and thicker for more battery life.