They could use technology. For instance, they could have special grappling hooks or stun devices. This would help them in combat and mobility.
By training their body to the extreme. They could become super - fit and strong, and also master various martial arts. Their fighting skills could be on par with or even better than some heroes with quirks. Also, they could study the quirks of villains and heroes alike, finding weaknesses and using that knowledge to their advantage in battles.
A quirkless vigilante in My Hero Academia fanfiction would likely have a very different origin story compared to the typical hero. They could have been bullied or discriminated against because of their lack of a quirk. This would fuel their desire to prove themselves. They might start small, like stopping petty crimes in their neighborhood. As they gain more experience and confidence, they could take on bigger threats. Their lack of a quirk would also make them more relatable to the readers, as most of us are 'quirkless' in the real world. And their journey from being an unnoticed figure to a respected vigilante would be a great story arc.
I really like the fanfic 'Izuku: The Quirkless Hero'. In this story, Izuku forms a special bond with some of the underdog characters. They work together to create a new style of heroics that doesn't rely on quirks. It also explores his relationship with All Might in a different light, as All Might becomes more of a mentor in non - quirk - related areas.
One possible story could be Izuku proving himself in combat training without a quirk. He could use his intelligence and strategic thinking to outwit his classmates with quirks. For example, he might study their weaknesses in advance and set up elaborate traps during the training exercises.
One great fanfic in this genre is 'Quirkless in a Quirked World'. It really delves into how Izuku, being quirkless like Saitama in OPM (in the sense of starting from a disadvantaged state), overcomes challenges. The story shows his determination and how he adapts Saitama - like training methods to become a great hero in the My Hero Academia universe.
Izuku could develop his own unique fighting style. He combines his knowledge of hero strategies and his physical abilities. Maybe he invents gadgets like Batman, which give him an edge in battles against quirk - users. This would make him stand out as a badass hero in the fanfiction world.
It could be a powerful villain who sees Naruto as a threat to his plans in the My Hero Academia world. Maybe a villain who wants to use Naruto's unique abilities for his own evil purposes and when Naruto refuses, becomes his enemy.
One common theme is the struggle for justice outside the traditional hero system. Vigilantes in these fanfics often feel that the official heroes are not doing enough or are too restricted by the law, so they take matters into their own hands.
One idea could be Naruto somehow ending up in the My Hero Academia world and using his ninja skills in a vigilante way. For example, he might use his shadow clone jutsu to create distractions during villain attacks, which is a completely new concept in the MHA universe.
The quirkless Izuku in fanfiction with OPM elements is a different beast compared to his canon self. Canon Izuku is full of hope and admiration for heroes. In fanfic, he may start off the same but as he trains like Saitama, he could become more focused on his own power and less on the hero image. His fighting style might also be more straightforward and brutal, like Saitama's. He might not care as much about the flashy moves and hero rankings, but more about being able to defeat any villain in his path.
Another great one is 'Vigilante Rising'. This fanfic focuses on the backstory of the vigilante. It explores how they became who they are and their reasons for operating outside the law in the My Hero Academia world. The action scenes are well - written, and the relationships between the characters are engaging.