I'm not sure which option is better
Sharing body AU
I'm a firm believer that Shen Jiu is not gonna be normal about Shen Yuan. In a scenario where they share a body, he would hate it at first. There is some kind of parasite in his mind, and he can't get rid of him. He hates it when something is out of his control.
However, it is also comforting? There is someone who can't leave him, who sees all his vulnerability and rot inside his soul, but he's still there, no matter what. He will stay by his side.
Even if they fight, Shen Yuan tingles something in his brain with his different worldview, especially as this brat believes that people aren't born bad. Too forgiving.
In the beginning, he wants him to be gone asap, so he tries to find different ways to get rid of him. But the more he spends time, the more he understands that he should stay he is too dangerous to let go. Obviously, he knows Peak Lord's secrets, so Shen Qingqiu can't let him do unguarded. He's safer here, with him.
The boy has so much vitriol but is still too naive to notice other people's intentions.
So we end here with Shen Jiu's weird situationship with Little Bodysnatcher and the weird codependency they establish meanwhile.