In this fanfiction, a possible plot could be that Naruto's chakra is mistaken for a new type of quirk. This causes a stir in the MHA society, and Naruto has to navigate through the new world while also dealing with those who want to either study or control his 'quirk', all while acting as a vigilante to do what he believes is right.
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.
Perhaps Naruto, with his strong sense of justice from his own world, decides to take on the role of a vigilante in the MHA world to help those who are being overlooked by the official heroes. He could start training some of the quirkless individuals in ninja techniques to give them a fighting chance.
A good plot for My Hero Academia Vigilante OC fanfiction could start with the OC finding a strange quirk - enhancing drug on the streets. This leads them to investigate a new criminal organization that is creating and distributing these drugs. As they dig deeper, they realize that this organization has ties to some powerful people in the hero world. Along the way, they might meet an old friend who has turned to the dark side and is involved with the organization. They have to decide whether to save their friend or focus on taking down the organization. Another plot could be that the OC's secret identity is at risk of being exposed when a nosy journalist starts following their every move.
One idea could be to create an OC vigilante who has a unique quirk that's not very flashy but extremely useful in stealth situations, like the ability to blend into shadows completely. This would add an interesting dynamic as they could move around unnoticed by villains and even some heroes. Another idea is to have the OC vigilante be from a family with a bad reputation in the hero world, so they have to fight against the stigma while trying to do good.
A cool plot could involve the sharing of techniques. Sasuke might teach Bakugo some of his sword skills, while Bakugo shows Sasuke how to better control his explosive power without overusing it. Also, the concept of the Ninja Academy in Naruto could be merged with UA High. Students from both could participate in a joint training session where they learn from each other's combat styles and strategies.
One idea could be that Naruto characters are transported to the My Hero Academia world and have to adapt to the different power system. For example, Naruto might be confused by the concept of Quirks at first but then find similarities to chakra. Another could be a training session where Deku and Naruto train together, with Naruto teaching Deku about his never - give - up attitude.
How about a fanfiction where the villains and heroes have to team up? There could be a greater threat, like an alien invasion or a powerful quirk - user who is out to destroy the entire world. The heroes and villains realize that they can't defeat this threat alone, so they put aside their differences. This would lead to some really interesting character interactions, like Bakugo having to work with Shigaraki and both of them learning to respect each other's skills despite their opposing natures.
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 idea could be a crossover where Naruto characters end up in the My Hero Academia world. For example, Naruto could meet Midoriya and they could learn from each other's determination. Another could be a story where the quirks in My Hero Academia are similar to the ninja abilities in Naruto. Maybe a character has a quirk like shadow clones. A third idea might be a time - travel fanfiction where a My Hero Academia character goes back in time to the Naruto era and has to adapt to the different fighting styles and cultures there.
First, for your My Hero Academia Vigilante OC in fanfiction, consider their motivation. It's important. Are they fighting for revenge, or to protect a certain group of people? For example, if they had a family member who was wronged by a villain and the heroes didn't do enough, that could be a strong motivation. Next, when it comes to their skills, don't just rely on their quirk. They should have some combat training or street - smart skills. Like being able to quickly assess a situation and come up with a plan. And finally, their relationships with other characters can add depth. Maybe they have a begrudging respect for some heroes while being at odds with others.
Maybe a plot where a group of Warhammer heroes are sent to the Hero Academia world to stop a chaos invasion that is starting to seep into that realm. They have to work with the local heroes, sharing their knowledge of dark magic and ancient weapons to combat the new threat. This would involve a lot of cultural clashes and learning experiences between the two groups.
An interesting plot could involve an OC who is initially thought to be quirkless but discovers a latent quirk later on. This OC could then struggle to catch up with the others in terms of training and development. Or, an OC with a shapeshifting quirk could be used in a plot where they have to infiltrate a villain's lair by taking on the form of one of the villains' minions. They would have to be careful not to be discovered and also gather intelligence along the way.