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.
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.
Sabo's done quite a bit. For one, like I said, he's climbed up in the Revolutionary Army. That's no small thing. It shows he's got the skills and the leadership. Then there's the Mera Mera no Mi. Grabbing that devil fruit was huge. It not only gives him power but also kind of ties him to Ace in a way. And in battles against the World Government, he's been standing strong, showing that he's a force to be reckoned with. His actions are all part of his bigger plan to change the world for the better.
Well, Sabo in One Piece. He's like this super cool character. Sabo was a noble, right? But he didn't like the whole noble life of oppression and stuff. So he left. And he met Luffy and Ace. They had this amazing bond. Then that incident happened where everyone thought he died. But he didn't! He got rescued and ended up in the Revolutionary Army. He's now all about fighting for freedom and justice, just like Luffy in his own way, but on a different path. He's got these great skills and is making a real impact in the One Piece world.
Luffy, of course. He and Sabo have a deep history. They grew up together and Sabo was like a brother to him. Their reunion would be filled with joy, hugs, and probably a lot of tears. Luffy would be over the moon to have Sabo back in his life and on his ship.
Perhaps in this fanfiction, Ace, Luffy and Sabo will make Marineford a place where pirates and marines can live in harmony. They could set up some rules that both sides can abide by.
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.
Their relationship could develop through shared adventures. For example, if they are on a mission to steal important documents from a World Government facility. They would have to rely on each other's skills. Sabo's ability to fight and Koala's knowledge of infiltration. As they face various obstacles, they start to trust each other more deeply, and their relationship evolves from comrades to close friends.
A third common theme is justice. Sabo is a character who fights for what he believes is right. In fanfiction, this can be shown in different ways, such as him standing up against an unjust regime or helping the oppressed. It reflects his values from the original story and is often used to drive the plot forward in an engaging way.