Ushimaru is older so it is hard to tell how much of that is age, but his face looks more rugged than either Ryuma or Zoro. From the limited picture we've seen Ryuma was pretty much identical to Zoro.
We saw Toki with no baby when she met Oden and there was no mention of it, doesn't add up and is needlessly complicated, and that doesn't explain why Ushimaru looks so much like either of them.
The explanation is in some combination atavism/strong genetics/ pseudo reincarnation/will inheritance of the one eyed Shimotsuki green haired swordsman, I'm not discounting the possibility that Ushimaru ends up being Zoro's dad, but he is more likely to be an uncle or great uncle (no literal twin stuff necessary, lots of the family just look very much alike) keep in mind Kozaburo's sillouete was also intentionally obscured and I'm betting he looks very very much like Zoro too.
We saw Toki with no baby when she met Oden and there was no mention of it, doesn't add up and is needlessly complicated, and that doesn't explain why Ushimaru looks so much like either of them.
The explanation is in some combination atavism/strong genetics/ pseudo reincarnation/will inheritance of the one eyed Shimotsuki green haired swordsman, I'm not discounting the possibility that Ushimaru ends up being Zoro's dad, but he is more likely to be an uncle or great uncle (no literal twin stuff necessary, lots of the family just look very much alike) keep in mind Kozaburo's sillouete was also intentionally obscured and I'm betting he looks very very much like Zoro too.