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.
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.
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.
One common theme is identity exploration. When the characters' genders are swapped, they often have to re - evaluate who they are and how they fit into the world. Another theme is relationship changes. Their interactions with friends, family, and love interests can be very different. For example, a male - to - female swapped character might have a different dynamic with a male friend now.
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.
Well, a common storyline in My Hero Academia demon quirk fanfiction is the origin story of the quirk. It could be that the character got the demon quirk from a mysterious experiment gone wrong. Then there's the idea of a character with a demon quirk having to hide it from society because it's seen as too dangerous. And of course, there are those stories where the character with the demon quirk has to fight against other demons or dark forces that are attracted to their power.
The 'Underdog to Hero' trope is also common. Even when Izuku has a new quirk, he often starts off being underestimated. He has to prove himself not only to his classmates but also to the heroes around him. This trope is great for character development as we see Izuku grow in confidence and skill.
He could have a 'Ninja Arts' quirk. This quirk would allow him to perform all the ninja techniques he knows from Naruto, like the Rasengan or Chidori. His Sharingan could be an additional aspect of this quirk, enhancing his ability to learn and adapt to other quirks he encounters.
Perhaps a 'Healing Aura' quirk. Given Sakura's medical skills in Naruto, in the MHA fanfiction, she could have a quirk that emits a healing aura. This aura could not only heal physical wounds but also have an effect on quirks that have been damaged or overused. It would make her a valuable asset in battles and in hero agencies, as she could both fight and heal her comrades.