He could have a 'Rage Quirk' that makes him stronger the angrier he gets, which would be in line with his character in Doom. When he's in a fight, his anger just builds and builds, and his physical strength and combat abilities skyrocket.
A quirk like energy absorption might be cool for Lucy. She could absorb different types of energy, such as heat, electricity, or even the kinetic energy of an attack, and then release it back in a more powerful form. This would give her an edge in combat and make her a formidable character.
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.
He could have a quirk like 'Enhanced Intellect', which would further boost his already incredible detective skills. This would make him able to analyze situations even faster and come up with strategies on the fly.
The Doom Slayer could be a mysterious figure in the My Hero Academia fanfiction world. He might be from a parallel dimension where his fight against demons is similar to the heroes' fight against villains. He could team up with the likes of All Might or Deku, teaching them about his unwavering determination. His presence would add a new level of intensity to the battles, as he doesn't hold back and is used to facing much more terrifying foes than the average villain in My Hero Academia.
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.
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.
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.
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.