What if Wado Ichimonji was never actually Kuina's sword at all? Why would Kozaburo and Koushiro let the sword go so easily? They're from Ringo, the Shimotsuki family are well known for being born with a sword and being buried with that same sword. Ryuma's sword being returned to his grave was so important that Hiyori was willing to give up one of her father's swords (Oden wasn't from Ringo so its not as big of a deal) without question to see it returned. The idea of just handing Wado Ichimonji over to Zoro (apparently some random kid who liked her a lot and thats all) without burying it with Kuina would be insane to them and totally out of the question.
But for real. What if it wasn't Kuina's sword? She never actually says it's her sword. In fact, no one but Zoro ever suggests it actually belonged to her. She just gets it from the dojo after Zoro challenges her to a fight with real swords, and Koushiro IMMEDIATELY hands it over when Zoro asks for it even though it would be insane and wrong in his culture to disrespect the dead like that. It just doesn't make sense.
Unless it's actually been Zoro's sword the whole time, because it would be just as insane and wrong to deny a man the sword he was born with if he's leaving home to pursue his dream. What if he dies and is buried without his sword? It's unthinkable to people from Ringo. He's obligated to hand it over in that case. This ultimately connects to Zoro being Shimotsuki royalty descended from Ushimaru the former Daimyo, which is another popular theory. But thinking about the sword got me thinking about Sunacchi too, and how Zoro learned it from Kozaburo, but never said it himself (because he has no need to "throw away his name and wits")
if zoro ends up facing his father shimotsuki ushimaru/admiral green bull, and he pulls some "zoro, you kept your sword all this time, i knew you would always be a shimotsuki" shit zoro will absolutely yell SUNACCHI and fuck his dad up for trying to ruin his memories and dream
