One common theme is Sabo's loyalty. Many fanfics explore how loyal he is to his friends and the revolutionary cause. For example, he would go to great lengths to protect those he cares about.
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.
There's a Sabo fanfic called 'Sabo and the Revolution'. This one is interesting as it shows Sabo's role in the revolutionary army in more detail. It includes his strategies, his interactions with the other revolutionaries, and how he uses his skills to further the cause. It's full of political intrigue and action, which makes it a very engaging read for Sabo fans.
One common theme is brotherhood. Since they are like brothers in the original story, fanfics often explore and strengthen this bond. Another theme is adventure. They are all adventurous characters, so fanfics usually involve them going on exciting quests.
I really like the fanfictions that involve Ace, Whitebeard and Sabo. They are such interesting characters. In many of these fanfictions, the relationship between them is explored in great depth. For example, how Sabo copes with having Ace as his 'brother' again after thinking he was dead. And Whitebeard's role as a father figure to them adds another layer of complexity and warmth to the stories.
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.
They're popular because fans love the idea of Ace and Sabo being there for Luffy's upbringing. It shows a different side of their relationship. We usually see them as comrades, but in these fics, they're like family, which is really appealing.
You can start by looking on popular fanfiction websites like Archive of Our Own (AO3). It has a large collection of One Piece fanfictions, including many focused on Ace, Sabo, and Luffy. Another option is Fanfiction.net. There are also some dedicated One Piece fanfiction forums where you might find some hidden gems of Ace Sabo Luffy fanfiction.