Their relationship in the fanfiction can be seen as a symbol of hope and inspiration. All Might represents the pinnacle of heroism, and by choosing Deku as his successor, he gives Deku the chance to fulfill his dreams. Meanwhile, Deku's determination and growth also inspire All Might to keep believing in the future of heroism, even in the face of his own declining power.
One possible plot could be that Villain Deku is on a mission to undermine the hero society and crosses paths with Hero Deku. They engage in a fierce battle where Villain Deku taunts Hero Deku about his ideals. In the end, Hero Deku manages to get through to a small part of Villain Deku's conscience, leaving him with some doubts about his villainous path.
One common theme is power transfer. Since All Might gives Deku his power, this forms a central part of many fanfictions. Writers often explore how Deku copes with this new power and the responsibilities that come with it.
Another plot could involve All for One somehow transferring a small part of his consciousness into Deku. Deku then has to battle with this foreign presence in his mind while trying to keep his friends and the world safe from the potential chaos it could cause. This would also lead to some interesting character development as Deku tries to resist the dark influence.
Villain Deku might start to question his villainous ways. Meeting Hero Deku could make him see the good in heroism. Hero Deku, on the other hand, could become more empathetic towards those who turn to villainy. He might realize that there are reasons behind it and not all villains are just 'evil' for the sake of it.
One common theme is empowerment. Female Deku often overcomes challenges in a different way than male Deku, showing her strength and determination. Another theme is relationships, like her friendships with other characters or potential romantic interests.
If Deku and All for One interacted, it could change the story drastically. For example, Deku might learn some dark secrets about the origin of One for All that All for One knows. This new knowledge could either make him doubt his path or give him new motivation to prove that his way of using power is right.
In fanfiction, a quirkless Deku often has to rely more on his wits. Canon Deku got One For All, but in fanfic, he has to find other ways to be strong. For example, he might study different fighting styles more intensively.
Toga could be an ally. Since she has her own complex relationship with the idea of heroes and villains, she might find Deku's transformation interesting. She could provide some of her unique skills, like shape - shifting, to help Deku in his villainous plans. Additionally, Twice might also join them because of his loyalty to Toga and his own sense of being an outcast in the hero world.
A possible plot is that Villain Deku has a plan to destroy a major hero institution, and Hero Deku has to stop him. But when they meet, Hero Deku realizes that Villain Deku was once like him, full of hope. This makes Hero Deku hesitant to fight at full force. Or perhaps Villain Deku has kidnapped someone close to Hero Deku, and during the rescue mission, Hero Deku tries to convince Villain Deku to change his ways. Also, they could be from alternate universes where the roles are reversed, and they are forced to interact and learn from each other.