AceSan (and SaboSan) soulmates au where ace and sanji have a whirlwind romance in alabasta when they find out they're soulmates Ace and Sanji leave each other the mark of an accepted soulbond as a promise to meet again in the future and to have each other forever. (Think of it functioning like a vivre card + ABO bonding, it can only appear between soulmates)
Ace's mark doesnt quite disappear on Sanji when he dies in Marineford. It burns and burns bright until it fizzles away, faded but still there. And Sanji continues living and training in Momoiro island, and he still unconsciously pushes towards the bond, but no one nudges back, it's basically radio silence—until after Dressrosa when he feels it burn again as if Ace is both saying a sad hello and a goodbye.
After Sabo eats the Flame Flame Fruit, he not only gains it's power but also the feelings, memories, and will of Ace. At first he gets REALLY confused whenever he sees Ace's memories and fantasies of a blond man in all states of undress and, he mistakenly and stupidly thinks Ace was fantasizing about him. The feelings were basically like "blond man, forever, wish alive, takes care of luffy, crocodile, prince, etc". It's dumb and Sabo shouldve realized and connected it that, of course, ace wasnt fantasizing a future where Sabo "didnt die"—forgive him, he saw the Straw Hats and didnt see any blond man around "taking care of luffy", and that the crocodile wasnt a literal crocodile they cooked from the jungles of their childhood.... mild panic aside, Sabo figures it out eventually—Ace had a consummated soul bond with a blond man somewhere close to Luffy, and that soul bond... If he truly wanted it to, could be his. It was as if Ace was saying "you'd love him as much as I did, please take care of him and give him a chance."
