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.
Use descriptive language to bring the quirk to life. When Deku uses his quirk, don't just say he punches harder. Describe the energy coursing through his body, the sound it makes, and the impact it has on the environment. For instance, 'As Deku activated One For All, a brilliant green light enveloped his fist, crackling with raw power. With a thunderous roar, he launched his punch, sending shockwaves through the concrete, shattering it into a million pieces.' This will make your fanfiction more immersive for the readers.
In some fanfictions, Bakugo can become more empathetic. For example, if he experiences a situation where he realizes the pain he's caused Deku over the years. His aggressive nature might soften as he starts to understand Deku better. Deku, on the other hand, could become more assertive. Instead of always being the timid one, he might stand up to Bakugo more firmly in fanfiction scenarios.
It can also expand the universe in terms of relationships. When Deku fuses with other characters, it can lead to different interactions with his friends, family, and teachers. For instance, his friends might be initially hesitant or overly excited about his new fused quirk, which can create various story arcs. Moreover, the fanfiction could explore how the fused quirk affects Deku's standing in the hero community, whether he is seen as more of a threat or a greater hope.
One cool quirk idea could be 'Shadow Mimicry'. The user can blend into any shadow and move freely within the shadow realm, also being able to mimic the actions of those whose shadows they are in. It gives a lot of stealth and surprise attack possibilities in a fanfiction.
One interesting fanfiction could be where Deku and Bakugo have to team up for a special mission outside of UA. They start off bickering as usual but gradually learn to trust each other more deeply. Another could be a story set in an alternate universe where they are not rivals but friends from the start, and they help each other develop their quirks in different ways. There are also fanfictions that explore their relationship in a post - war scenario, dealing with the trauma and how they support each other through it.
Overusing his quirk is a common reason. Deku often pushes himself to the limit with One For All, and it can take a toll on his body. So in fanfiction, this could be a simple yet effective way to have him pass out.