One of One Piece's most popular arcs is the Marineford arc, which was the pinnacle of the first half of the story. The Marineford arc was a part of the Paramount War Saga which was centered around Ace, Luffy's brother. Ace was handed over to the Navy by Blackbeard and the organization subsequently announced his execution.
To save Ace, Luffy rushed to Impel Down, however, was unsuccessful in getting to his brother in time. Thus, the stage was set for the Paramount War and here, the mightiest would then go on to clash, with Luffy being thrown into the midst of everything. The Marineford arc was special and more ways than one and fans hold it close to their hearts to this day, with many believing that it is still One Piece's very best arc. However, this arc would have been very different had Oda not changed his plans for it at the very last moment.
Oda's Initial Plan For Marineford
Oda's initial plans for Marineford were very different. When Ace was transported from Impel Down to the Marine HQ, Oda had not yet made the decision that would later on completely change the course of Ace's story. When Ace was first introduced, Oda made it clear that he was Luffy's older brother. Slowly, fans also met Luffy's other family. After the events of the Enies Lobby arc, it was made clear that Garp was actually Luffy's grandfather despite working in the Marines and they shared a hearty and healthy relationship with each other, for the most part.
True Garp, Luffy also found out about his dad. Garp revealed that Luffy's dad was the leader of the Revolutionary Army and one of the most dangerous men in the entire world, Monkey D. Dragon. As such, the story started focusing more on Luffy's family. Soon after, Garp was thrown into the picture a bit more than usual and Dragon had quite a bit revealed about him as well. Naturally, fans would assume that Ace is also Dragon's son. This was hinted at especially when Garp met Ace. During their short-lived conversation when the latter was an Impel Down, Ace implied that he did not want anything to do with his father and that he only took the name of his mother when he set out to the sea. This implied that Ace held a grudge against Dragon, however, little did fans know that Ace's father was someone else entirely.
How Oda Changed His Plans For Ace
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Before beginning Marineford, Oda asked his editor how he would feel if Ace turned out to be the son of the late Pirate King, Gol D. Roger. The reaction he got from his editor was good enough for him to go ahead with this decision. This means that Oda initially did not intend for Ace to be Roger's son at all. In fact, this was a last-minute decision from Oda and it completely altered the story of both the Marineford arc as well as the character of Portgas D. Ace. This information was made public in One Piece shortly before Ace's execution. Sengoku announced to the entire world how Ace was brought up. He revealed to the world that Rouge managed to keep him in her womb for 20 months. It was also revealed that Ace was actually born in the South Blue on an island named Baterilla. He was eventually found by Garp, who had promised Roger that he would raise his child since the child was sinless. Garp eventually took him Ace and had Dadan raise him when he was too busy with his duties as a Marine Vice Admiral.
While this storyline made perfect sense and was certainly phenomenally well written, it cannot be denied that this was simply something that Oda came up with at the very last moment. Before Marineford, Oda had decided to stick with the fact that Ace was the son of none other than Dragon as it only made sense at that point.
How Marineford Would Have Changed
Had Oda stuck with his original plan, Marineford would have been very different. It would also have been very difficult for him to kill Ace off had he not been Roger's son.
Whitebeard rushed to Ace's aid since he considered him to be his son, however, had Ace been the son of Dragon, everyone would have expected Dragon to show up as well. This is backed up by the manga itself, where Ivankov states to Luffy that Dragon will never let his son die and definitely make a move. This is when Luffy reveals to Ivankov that Dragon is actually not Ace's father and that his father is Gol D. Roger. This comes as a surprise to Ivankov, however, had Ace actually been Dragon's son, fans would definitely have seen the Revolutionary Army at Marineford.
The Revolutionary Army would then have joined forces with the Whitebeard Pirates and beating two of the major forces of the world together would have been impossible for the Navy, strong as they are. This would mean that killing off Ace would be nearly impossible for Oda and that is likely why he decided to make him the son of Roger. Had Ace survived Marineford, Luffy would likely not have gone through his timeskip training and not grown as strong as he is today. Oda would have to find different ways to make him stronger and change the course of the story forever. Of course, it is always fun to speculate about what could have been, however, in the end, Oda decided to make Ace the son of Roger and kill him off so that Luffy would end up growing as a pirate.
Sabo In Marineford
One of the most intriguing aspects of the story that fans would have been able to see had Ace been the son of Dragon instead was the reunion of the three brothers. Sabo was already a member of the Revolutionary Army by the time Ace's execution was announced in the Marine HQ. Dragon would eventually have made his move and showed up to fight against the Marines and join forces with Whitebeard. At the same time, fans would also have seen Sabo come to Marine HQ. This is where the three brothers would then have reunited for the very first time since their childhood. Undoubtedly, this would have been an incredible moment for the fans to witness.
This would also have affected Shanks' arrival on Marineford. If the pirates and the revolutionaries would have won the war, there would not have been a reason for Shanks to arrive. In fact, Shanks' arrival at Marineford was also a last-minute decision that Oda took. Clearly, if there would have not been a need for it, Shanks would certainly not have shown up.
This just goes to show that one simple decision by Oda completely altered how Marineford played out. Fans are certainly grateful to Oda for having written write such an incredible story which ends with heartache for both Luffy and them alike, however, it is very interesting to see that this arc could have been completely different had Oda decided to stick with his original plans.