Well if completely what I mean, I'm challenging anyone who wants to try to write a crossover with One piece and Supernatural, for this crossover the story will start really just in the Marineford war, can have some kind of mentions before it but this is the initial point of the story.
The plot of the story is kind sample, the tenryuubito are demons, evils or whatever really motherfuckers supernatural you can think and the D's will be hunters, descendents of nephlim(sons of angels and humans) in the time that the gorosei was created and all the hunters was going killed some of them survived and adopted the name "D".
For this story Ace(and maybe whitebeard if you really want) needs to survive and who will save him(or they) will be a girl Winchester, daughter of a Winchester, a main character or an oc if you want, and the angel Castiel, she will be really importantant for the story because to open the gate of hell they need blood of a pure nephlim, half human ans half angel, you can create other Winchester like cousins or little brothers/sisters and you can joy the real Winchesters to the fight but in the beggining of the story Castiel will be in the Hell after be killed by one Tenryuubito trying to protect her and this will be why she hates so much demons.
You can make any important character that you want know the real face of the Tenryubito but the important ones will be Shanks, Rayleight and Buggy because they was there when Gol D. Roger arrive in Raftel, the last thing is that Pluton, Uranus and Poseidon will be massively weapons created to destroy demons.
just something to get you inspirated:
"Hello Ace, How are your fucking ass today?" Asked a girl entering in a room that has a 20 years old man in bed.
"Why…why are you helping me? I'm just a demon, son of a demon"
"Holy crap no! Believe me when I say that you're not a demon, I already saw real demons and killed them all, if you was a real demon I wold have shot you already and you wouldn't be even alive"
"But I'm still a pirate, everyone hates them, why are you helping me?"
"Well you can say that I don't care about this complicated stuff, you're just a man living your life, maybe you killed a lot of people like marines but they killed a lot of your people. Is kinda fair, don't you think?"
"You're thinking is fair that a lot of innocent people die?"
"Don't put words in my mouth, that is not what I mean. I'm saying that marines killed a lot of your people under the reason of justice and your people killed a lot of marines, maybe to be free, maybe to survive or maybe just because they're just freaks motherfuckers. Anyway, everyone have their reasons and fight for them, I'm not saying that they're right but is just how the world is, full of shades is grey but I can't just forgive that sons of bitches like demons, monsters or poltergeist that kill a lot of innocent people without any reason that's why hunters exist"
"What are you trying to say?"
"What I'm trying to say is that you have a important job, Ace, you and and all of the D's spread all over the world, a job to fight against the world government, in other words the Tenryuubitos"
"What? What do you mean with that?" "You're really slow, aren't you? I mean that you and the others D need to fight against the Tenryuubito"
"But why the D's? And I know that the Tenryuubitos are a bunch of motherfuckers but why we need to kill them?"
"Because they're not humans, they're demons that was send to control this world and the D's need to destroy them because they're the last hope of this world, the last bloodline of hunters and descendants of nephlins, half angel and half human"
Have fun!