I have the following Scenario in my head since a few month. Discussed it Long ago with
@playa4321
It begins with Luffy wanting to fight Kaido and he charges for Kaido, gives him one hit. Kaido gets a different from the hits he got before by Luffy, but nothing serious to him. So he counterattacks and Luffy is Pretty much damaged again. Difference from before, he doesnt faint instantly, he is able to move a bit, but is hurt alot. While he wants to charge again, someone Interrupts and charges for Kaido. It is Kid. He attacks straight on, but gets a Pretty similar treating to Luffy. While kid was able to dodge some attacks, he buyd (unplaned) time for Luffy to Regenerate, which was supported a lot by Marco and Chopper. So when Kid goes down, Luffy is charging in again.
These single faints, switching each other Always while the other one get healed goes for some Hours. Hours which make Kaido tired, but also Marco. Also Kaido realizes that each of his Opponents, Luffy and Kid, get a bit stronger with every fight, a bit more technique inside and a bit more strategy against Kaido. He doesnt worry, cause it would take days or weeks like that to get him down, even tho he gets more tired. While Kid and Luffy Always switch, they mock each other, telling the other one they will get him down and the other one should stay back on his butt.
So some Hours go into the land while they fight and fight. But Marco gets tired, the healing is Pretty much exhausting for him, the Phoenix-Power takes alot from him. So after he got a partly healing for Kid, suddenly he faints. Kid is able to stand up, but is exhausted a lot, cause he didnt got the "full package". But Luffy goes down, so he is in charge again,. He wants to go on, but gets interrupted by Killer and Zoro. Luffy gets thrown by Zoro to Chopper and tells Chopper he Needs to get Luffy fit as fast as possible. He trust him About that. Then Killer steps to Zoro, takes into Position. Kaido is laughing About These two, telling them they underestimate him, even their captains werent able to take the fight on him. But Zoro smiles. They suddenly both share the same words:
"We dont Need to."
So they charge at Kaido. Both not able to beat him, but able to buy time, to take him away from their captains. Luffy and Kid see how they fight, they see both are able to fight together in a Team and can overcome their own strength. Zoro gets a small Flashback in his head, where Killer explains his plan to Zoro, to Show the captains how strong a team-up can be, then a yonko is way to strong for each of them separately. While Zoro refuses first, he is able to accept the plan when he saw how Luffy got beaten. Also, Zoro and Killer are still wounded from their fights before, but they got some rest. Still way not enough, but enough for some minutes.
Meanwhile, Chopper is able to heal both alot. Killer and Zoro laying on the Ground and Kaido charges for his final blow, a firebreath. But the blow gets blocked by Luffy and Kid. Kaido gets Angry in the Background while Luffy and Kid discuss.
"Obviously you cant take him down alone, i am gonna help you weak Monkey!" - Kid
"If you Need my help to beat him, just ask Spikeyhead." - Luffy
(Notice here, they Keep the surnames, but dropped the insulting part ("Bastard" and "Idiot")
So they came to that Point and Kaido charges for them. He is gonna attacking them in rage, but when he arrives, he gets a strong hit from both and gets smashed far back.
"We have a talk here!"
Then the real fights beginns. A Long fight, but they are able to fight better in Team from Minute to Minute. Kaido goes in rage, changes to his zoan form and even in his mixed form, at his maximum of strength. He keeps Fighting them, throws soro breath attacks at them . It is a close fight and Luffy and Kid are going to get exhausting and get beaten.
Meanwhile Marco was able to wake up again and is watching at the result of Choppers healing. He smiles and tells Chopper he is highly impressed by his skill, specially because he doesnt even have some DF for it and is just using his Knowledge. He says: "Your captain is Incredible lucky to have a Doctor like you with him."
Back to fight, Kaido is giving them the hardest time they ever had, in his mixed form, they are hardly able to hit him, but they got some injuries at him. After he throw another soro breath attack on them, there is a lot of fire, gas and sand in the air. So he cant see them for a few seconds. Kaido declares he has enough of this stupid fight and he will kill both of them here and now. He is going to give them a final hit. So he is preparing some attack for himself and Looking for them in the Gas/Fire/Sand in front of him. Then finaly he can sense their Energy/haki, but it is not in front of him, they are up in the sky, way above him.
Up in the Sky, Kid is Building a HUGE fist, out of all Metall going around in Onigashima, even the Sword from the entrance is raising (which gives Panic to a lot of People). Right next to it, Luffy is growing an more or less same size fist. While alot of metal is going around Kid and his metal hand, he also starts to get a huge amount of it round Luffys Hand, because they are not "fist at fist". At the same time, Luffy is heating up his Hands, bring is on fire, which grows even over Kids fist. While still growing both, they charge down at Kaido, Screaming the attacks:
Luffy: "FIRE GOD..."
Kid: "METAL ZILLA..."
LuffyxKid: "GUN!"