It could be a rogue scientist. They might be jealous of the natural quirks that people are born with and decide to steal them to create artificial super - beings. They would be highly intelligent and use their scientific knowledge to develop devices or techniques to steal quirks.
An ancient being that has awakened from a long slumber. This being has the power to steal quirks and sees the modern quirk - users as a source of power. It has a different set of values and morals compared to the heroes and villains of the My Hero Academia world, making it a very unpredictable and dangerous antagonist.
Perhaps a disgruntled hero who has turned to the dark side. They feel that they were never given the proper recognition in the hero world despite their powerful quirk. So, they start stealing other heroes' quirks to prove their superiority and make a name for themselves in a more villainous way.
One idea could be a villain who can steal quirks and is secretly a former hero trainee who was wronged. They start stealing quirks to create a super - powerful army to take revenge on the hero society that failed them.
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.
First, consider their motivation for using the Copy Quirk. Are they using it to become the best hero? Or maybe to prove themselves to someone? For example, if they want to be like their idol hero, they might copy only the quirks of that hero at first. Then, think about the limitations of the quirk. Maybe they can only copy a quirk for a short time, so they have to strategize when to use it. This would force them to study different quirks and opponents carefully.
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.
His quirk could be something like 'Limitless Rage - Transformation'. When he gets angry, his body transforms into the Hulk form, growing in size and strength. The angrier he gets, the more powerful he becomes, similar to how it works in the Hulk's original story.
Perhaps an all - powerful villain from another dimension invades. This villain has the ability to control the minds of those with quirks and uses them to fight against the remaining heroes. He wants to take over the 'My Hero Academia' world and remake it in his own image. His powers are so strong that it takes all of the heroes' combined efforts just to try and figure out a way to stop him.
A group of villains who specialize in causing chaos during disasters could also be the main antagonists. They would constantly try to undermine Backdraft's efforts to keep the city safe during fires and other emergencies.
One great Izuku fanfiction is 'A Different Path'. In this story, Izuku gets a quirk that's not One for All but something entirely unique. It shows his journey of self - discovery and how he still manages to be a hero in his own way. The author does an amazing job of exploring Izuku's character and his relationships with other characters like Bakugo and All Might in this new scenario.
The 'Plant Symbiosis' quirk would be a great addition to a fanfiction. The individual with this quirk can form a symbiotic relationship with plants. They can communicate with plants, make them grow rapidly, and even use plants as an extension of their body for things like creating shields or lashing out at enemies. This quirk could bring a whole new element of nature to the story.