There are many ways to handle this in fanfiction. Some might write that a powerful artifact is found. This artifact has the power to restore life. The Straw Hats or other relevant characters then go on a quest to find this artifact and use it to bring back Ace or Sabo. It could also be that there's a prophecy in the fanfiction world which foretells the return of the fallen, and the characters work towards fulfilling the conditions of that prophecy.
Fanfiction writers often play with the idea of unfulfilled dreams or destinies. Ace and Sabo had so much more they could have done. So, in some fanfics, their memories are so strong that they exist as a sort of spiritual force. And this force is then able to influence events in a way that leads to their physical resurrection. For example, their memories might guide Luffy to a place where he can find the means to bring them back.
One of the main attractions of this type of fanfiction is the chance to delve deeper into the characters' psyches. Ace and Sabo have complex backstories, and when we add the 'Back Scream' concept, it's like opening a Pandora's box of emotions. For instance, we might see how Ace's memories of his mother influence his actions, and how Sabo's memories of his noble family's rejection shape his view of the world. These memories, with the added intensity of the 'Back Scream', can create a very engaging story.
One possible fanfiction could be that Ace and Sabo meet again after a long time. Sabo starts to have fragmented memories of their past adventures together with Ace and Luffy. As they interact, more and more memories flood back. They recall the times when they were training hard, the pranks they played on Dadan, and the dreams they had as kids. This reunion could be filled with both joy and a touch of sadness for the time they lost.
In an 'Ace Sabo Memories: Back Scream' fanfiction, the story could center around a mysterious force that makes them relive their most painful memories. This force could be related to an ancient devil fruit power or some sort of dark magic. Ace might remember the time he was rejected by his father, and Sabo could be forced to recall the moment he thought he lost his brothers. The 'Back Scream' then becomes their internal struggle against these overwhelming memories.
The plot could center around Sabo's journey of self - discovery as his memories of Ace return. He might realize that there were parts of his identity tied to Ace that he had forgotten. For example, their shared love for freedom and adventure. With these memories back, he could make different decisions in his current life. Maybe he would be more willing to take risks or be more protective of Luffy, seeing as Ace was always so protective.
One great 'Ace Sabo Luffy Fanfiction' is 'The Brothers' Bond'. It beautifully explores their relationship, showing how they support each other through various challenges. Another one is 'Sabo's Return'. It focuses on Sabo coming back into Luffy and Ace's lives and how they all readjust. There's also 'Ace and Luffy's Adventure', which takes the three on a fictional journey filled with action and heartwarming moments.
Another interesting one is 'Sabo's Return: With Ace'. In this story, Sabo returns much earlier than in the original story. It details how he and Ace reunite and the impact it has on the Straw Hat crew and the overall One Piece world. The story is full of action and heart - warming moments between the two.
A great Sabo meets Ace fanfiction might be the one where they meet in a parallel universe. In this universe, things are different but their brotherly love remains the same. They learn about the differences between their two worlds and how to make the best of their reunion.
One possible plot could be that Ace has been through some tough battles and has been missing Sabo for a long time. When they reunite, there are immediate hugs and tears of joy. They then sit down and share all their adventures that they had during their separation.
Well, one possible future for them in Marine fanfiction could be that they form a special unit. Ace with his fire power, Luffy with his rubber abilities and Sabo with his skills would be a formidable team. They could be sent on special missions to deal with the toughest pirates. This way, they are still following their dreams of adventure while being part of the Marines.