I love this shit so much you have no goddamn idea. It was a staple in my pre-one piece days to have like, biting rituals and whatnot with permanent marks for both parties so I'm still very very much into that.

Collars are a different thing entirely though. Apparently they're very polarising. I love the idea of Sanji wearing something like that under his clothes (the shirts would hide it so well!!) but funnily enough, it's Zoro I'd rather see in a collar and leash :3 A big thick leather one with a visible buckle would be lovely for him. Sanji's the type I'd give sleek elegant designs to because he's a stylish boy but Zoro is more... punk? I guess? Chunky 'in-your-face' stuff suits him a lot, imo.

Visible signs of mutual ownership can be hard though, cause like... I love the idea of Sanji sporting one of Zoro's earrings but am not sure what the equivalent of that would be, y'know? One day maybe I'll think of it but I'm drawing a blank other than marks for him :(