One interesting plotline could be Deku having to face off against his classmates in a series of challenges that test not only their quirks but also their teamwork and strategic thinking. Maybe there's a misunderstanding that leads to the conflict.
Deku himself is likely to shine. Given his powerful quirk and his growth throughout the main series, he'll surely put up a great fight against his classmates. Bakugo could also shine as he's always been in competition with Deku and has a very strong quirk.
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.
There's also 'Growth Together'. This fanfic shows how Deku and Todoroki support each other during tough times at UA. They face challenges not only from villains but also from within the school. Their relationship develops as they help each other overcome these obstacles.
One idea for a Deku's Quirk fanfiction could be exploring what would happen if Deku's quirk manifested in a different way. For example, instead of having to gradually control One For All, he suddenly has full mastery from the start. This could lead to a very different story arc where he is immediately seen as a top - tier hero - in - training, causing envy and admiration in equal measure among his classmates.
One idea could be a fanfiction where Deku's quirk suddenly mutates. Instead of just having One For All, he starts to develop an additional power related to it, like the ability to share a fraction of One For All's power with his friends temporarily to boost their own quirks in a time of great need.
There are fanfictions where Deku teams up with unexpected characters. For example, he might partner with a minor villain who has a change of heart. This creates interesting dynamics and new story arcs that deviate from the original plot but still stay true to Deku's character.