“You are mine.”
This is just my wishful thinking but when I think about Reverse Bill (Will) and Dipper, I like to think of Will still keeping his demonic ways. Don’t get me wrong, crybaby!Will is adorable but I imagine Dipper and Will’s relationship to be a bit more, unstable and unhealthy than the original headcannon. Sort of like a dark version of a love-hate relationship.
Imagine Will resenting Dipper from the very beginning and he still does but the longer he is in contract with the psychic, the more dependent he becomes. He hates Dipper’s strong hold of him and at times, he’d like to imagine a scenario where he’s the one to kill Dipper with his own hands (which he cannot actually do due to the contract, this frustrates him even more). However, there are also times where he’d kill for Dipper and desperately wants Dipper to be his as much as he is Dipper’s.
Now Dipper’s part in this is a bit more simple. He knows Will’s conflicting emotions and is even consciously encouraging it with the way he acts and regards his pet. With that, he has the demon where he wants him, taking advantage of Will’s devotion and shockingly enough, enjoying Will’s hatred for him. I have this notion where Dipper is a complete sadomas (displaying both sadistic and masochistic tendencies) and could actually be a power bottom in the event of being sexual with Will.