Ace dying at marineford but instead of what he told Luffy in canon he just cried and said that he never thought he could die with so many regrets in his heart.
Ace was strong, his devil fruit was fighting to knit his body back together, and it gave him time to talk.
So he talked, he told Luffy every promise he made that he's breaking by dying there. Luffy tried to cut in & reassure Ace that he'll be able to keep them cause he won't let his brother die. Ace laughed and told him to shut up & listen to his big brother for once.
Ace is forcing himself to speak, to let Luffy take on the promises that he had made, so Luffy shut up, he listened.
Luffy gulped back a sob as his big brother's hand cupped the back of his head and Ace spoke, sounding so weak and breathless that Luffy could hardly believe that this wasn't all a horrible nightmare.
His brother spoke about the Whitebeard Pirates and how he fell in love with the crew, how the other commanders had become siblings to him. How he's worried about Marco now that both Thatch and himself were both dead or dying, Marco's so sensitive about crewmates dying . Ace told him to keep an eye out for them if they need help in the future.
How BlackBeard had betrayed Pops and everyone in the crew by killing Thatch, Ace explained that he chased him around the GrandLine in a fit of rage and finding out that the traitor had decided to go after Luffy, since the teenager had been causing a surprising amount of trouble for the marines as a rookie, had sent Ace spiraling in rage which had resulted in him getting caught.
Some of the Whitebeard Pirates, backed up by both Jimbe and Trafalgar Law, managed to start dragging the two brothers back to the Moby Dick, where Whitebeard himself was batting away marines as he kept his eye on his youngest son.
Ace coughed up blood in his throat and spat it out as he tried his best to clear the mangled mess of his chest so he could keep speaking; "I wish I got to say sorry to Sabo before he died, I don't think I ever once apologized to him. I swore on his grave that I would watch after you, and now I've failed on that front too, I'm gonna have hell to pay when I meet him later on." Luffy couldn't help the sob that spilled out as Ace rubbed his thumb over the secret little ASL tattoo that Luffy had at the base of his skull. "Keep my knife Lu, I know you keep one of Sabo's old fountain pens. Keep it on you, use it to keep yourself safe since I can't anymore."
Ace told him briefly about the situation in Wano, and how he'd met an adorable little girl there named Tama that had reminded him so much of a little Luffy and a younger Sabo that he couldn't help but be charmed. How the whole of Wano was starving to death because of Kaido. How the Creature had the entire country hostage, and he'd promised to come back and help when he got the chance, and now he would never be able to save the little girl that reminded a younger homesick Ace of Luffy.
Law and the Whitebeard field medics are fussing over Ace's back, calling for another doctor to help support Law, and Marco to heal him but Ace is focused on Luffy, their foreheads pressed together as he kept talking, though he should have long run out of breath. It is of course The Will of D, that stubbornness that all D's possess, allowing him to prolong his death so he can keep speaking to his baby brother.
He told Luffy that he wished he had gotten the chance to go back and visit Makino and Dadan, that he missed the two of them sometimes when he was on an island and sees kids with their mothers. "Luffy, do you think you could take my hat back for a visit sometime? I know it's a weird request but I think it would be good for Dadan."
The hand that wasn't holding his little brother by the back of his head wrapped around that skinny body and he sighed softly as he hugged his little brother harder. "Tell Shitty Gramps that I'm sorry that I died, but that I was happy that I went like this, protecting my cute baby brother. After all that's my job as your brother Lu, I would die for you a thousand times over. Grandpa's gonna blame himself but make sure he knows that I wasn't too upset, tell him that I love him and forgive him. Don't let him break down over this Lu, it's the risk I knew about and I took anyways when I set sail under a black flag."
Then at the end with his body finally starting to give up, Ace smiled and finally started to cry "Luffy I'm so sorry I didn't get to see you reach the top but you've always been my Pirate King. My dreamer of a little brother, I know you'll do it. I'm so proud of you Luffy. Thank you so much for loving me all these years! Thank you for being my brother and wanting me to live!! Thank you for trying to save me. Thank you for being born after me Luffy so I could be your big brother in this world."
Ace died with a smile on his face and a hand cupping the back of his brother's head, fingers tangled into dark locks. Luffy cried and screamed his heart out but he carved Ace's words into his soul. He'll take all those regrets to heart and fulfill every promise that Ace made.
Whitebeard dies, having waded into the battle to grab a few of his more vulnerable children who had been having a hard time retreating to the ship after he'd called out that they'd gotten Ace. BlackBeard killed him, and it was another strike against the man in Luffy's eyes.
One of the things that Luffy was remarkably good at was keeping the promises that he made to others. It had been something that Makino and Garp had taught him when he was really little, that your word was your honor and your bond.
It took him far longer into his two year training break than he wanted to admit that Ace's death and regrets had broken him a little bit. That the little part of him that had still believed that Ace was the coolest, strongest and best person ever had taken a large toll on his heart. Even after Jimbe reminded him that he still had his Nakama, Luffy had still felt lost and very alone in the world.
Ace had fallen victim to both his own pride and the fist of the red mutt of the marines. It had been a horrifyingly brutal death, but thinking back on it later it wasn't surprising. Ace had always been hotheaded.
Luffy was the best of his brothers at keeping the promises he made. He had never in his life made a promise that he couldn't keep with a little determination and a lot of hard work.
So he fulfilled Ace's regrets.
He let the Whitebeard Pirates know the extent of Ace's love, and specifically sought out Marco, Izo, and Haruta to let them know that Ace had loved them all so much. That he was so thankful had he had gained such an amazing family. (He barely managed to apologize to Marco for Ace's wildness and recklessness and he had to skip out of the way of an angry smack.)
He managed to get ahold of his grandfather after his two year training session with Rayleigh. Garp had semi-retired from the Marines, only staying on in the sense that he was in charge of training and recruitment. He had very nearly killed Akainu in a fit of rage after the man had punched the fatal hole through Ace's chest, and he had been lucky not to get thrown into a cell himself. Luffy relayed everything to Garp that Ace had asked of him. He told his grandpa everything that Ace had said about knowing the risks of sailing under a black flag. He got to watch his gramps cry and he stepped into the man to give him a hug.
When he met a miraculously alive Sabo he cried and cried into the older man's arms. His heart ripped to pieces all over again even as that old dull ache in his chest, caused by Sabo's death healed over. Sabo managed to win Ace's Devil Fruit and after everything that happened in Dressrosa, Luffy sat Sabo down and talked to him about Ace's death and the final regrets their brother had. He fiddled with the dagger he had been given that had belonged to his brother as he spoke and he let Sabo cry over the apology that he relayed for their brother.
He took care of Tama and liberated Wano, it was hard and it took way too many fights but he managed to take down Kaido and opened the borders just like Oden wanted. He got to watch Tama eat a few good filling meals before they left and he made sure to have Sanji cook her a massive pot of rice so she could eat as much as she wanted in return for the bowl that she had made for him. They left bag of gold with her, so she could take care of herself for a time and take a break from weaving hats everyday and focus on her ninja training. As he left he planted a flag with his Jolly Roger by the diminished waterfalls and declared for the world to hear and see that Wano is under the Strawhat's protection. He left with Yamato who joined them for a short time so he could see the world outside of Wano's borders.
When he's declared a Yonko he threw his support behind Marco, who had become the captain of the New Whitebeard pirates. They entered into an alliance purely based on the fact that they'd shared a brother. Whitebeard's final command had been an order to back the Strawhats up and one of Ace's last requests of Luffy had been to back up Marco.
He declared to the world that the New Whitebeard Pirates were off limits, and were his allies and brothers in the same breath. Luffy says it when he answers a call to arms against Blackbeard and delivers the killing blow after the traitor went after Marco with a Seastone sword. He pulled the phoenix off the ground and to his feet, covered in the monster's blood and when Marco thanked him he said, "Of course, you're my big brother too." The battlefield went silent for a moment, and by the next day, the whole world knew that the Yonko Strawhat Luffy considered Marco the Phoenix his older brother.
When he finally got a bit of free time, he took Ace's hat from his grave with a cheeky wink and ran out of the graveyard, then he ordered his crew to sail straight over the calm belt and into the East Blue. He had a promise to fulfill after all. It didn't take them too long to get to Dawn Island and Fuusha Village. When they docked he gave everyone free time but told them to behave themselves because this was his hometown and he didn't really want them wrecking the villagers' houses.
He stepped onto the solid land of his hometown for the first time in years and he couldn't help but smile a bit sadly. He carried Ace's hat in his hands. Scarred fingers ran over the faded orange leather over and over again, Luffy worried at the hat just like Ace had. Some of the color had been bleached out of it because of how long it had been hanging off of Ace's gravestone.
He walked into Fuusha Village with a thoughtful smile on his face and headed straight towards Partys Bar.
The reunion with Makino was teary eyed and Luffy couldn't help but laugh loudly, his signature Shishishishi snicker rang throughout the village and people stuck their heads into the bar to say hi to him. WoopSlap came in to yell at him too, and the pirate couldn't do anything but laugh as the old man hit him over and over again with his cane.
Kids that had been little more than toddlers who clung to his and Ace's legs before they both left came in screaming that Luffy was back to visit. He played with them in the main street, tossing a few of the braver boys up into the air and letting their screams of delight ring in his head as he played with them in an attempt to make his way across the village to head into the forest.
Dadan was harder, the old bandit had always had a lot of pride. She had also raised Ace up from a baby and had loved him more then she would ever willingly admit. So he walked his way into her lands and was pleasantly surprised that none of the animals that would have attempted to eat him came anywhere near him. He did hunt down a bear or two for the older woman, wanting a peace offering as he pushed his way into her home for the first time in years.
He laughed loud and obnoxious, he threw open the door and stepped in like he owned the place, he dragged his catch in with him against the screaming protests of the bandits inside. He glared at two of the newer members and they got to work handling the bears that he had caught.
Dadan was both delighted and absolutely furious to see Luffy in her house for the first time in years. The future pirate king allowed himself to be beaten, it didn't hurt and it let Dadan get a bit of frustration out before he had to break the woman's heart.
Luffy sat her down and fiddled with the brim of Ace's hat, placing it gently on it on the table in front of her and throwing himself into a chair, "He asked me to bring this back for a visit before he died. I promised that I would, Ace also told me that he regretted not coming back to visit you and Makino while he had the chance. Thank you Dadan, for raising us the best you could. Thanks for being the closest thing we ever had to a mother."
The old bandit reached out and brought the hat to her chest and started to shake as the tears poured from her eyes and Luffy leaned in and pressed a hand to her shoulder as he looked at her so gently. "Thanks Dadan, he really loved you a lot."
He eventually became Pirate King and went back to sit at the base of Ace's grave, dagger on his belt and Boshi hanging low, shadowing his face as he poured Sake into the cups that Sabo had left there years before and smiled. "Hey Ace. I took care of everything for you. Let me tell you all about it…"
