Tbh, I'm not convinced Yamato is "good". BM had 70+ children, there were always grounds for some of her kids to be actually good, especially with the ways she treated some of them.
Yamato ALSO being someone who betrays his parent (the only one) just seems like a retread. I feel like the mask is hiding some important identity.
Idk, we'll see. We have to rule out the PH design first.
Edit: Basically, I'd rather him be a potential Strawhat or bad, rather than just an arc ally poised to support Luffy here.