One possible plot could be that Izuku's friends discover his kidnapping and start a rescue mission. They might face various obstacles set by Overhaul and his minions while trying to find Izuku's location.
One interesting Lady Nagant x Overhaul fanfiction could be a story where they form an unexpected alliance in the criminal underworld. Maybe they start off as enemies but then realize they have common goals. For example, they might both be trying to take down a bigger, more powerful villain in their world.
One idea could be exploring Overhaul's backstory in more depth. Maybe show his childhood and what led him to become the villain he is. Another could be a redemption arc for him, where he teams up with the heroes against a greater evil.
You can start by looking on popular fanfiction websites like Archive of Our Own (AO3). It has a large collection of fanfictions from various fandoms, and you're likely to find some Lady Nagant x Overhaul ones there. Just use the search function and type in their names.
All Might could also be a potential hero to save Izuku. He has a strong bond with Izuku and his presence alone could intimidate Overhaul. He would use his experience and power to rescue Izuku from the dangerous situation.
One idea could be to explore Overhaul's backstory in more detail. Maybe show how he became so obsessed with 'perfection' and order. For example, you could create a scenario where he had a difficult childhood in a chaotic environment, which led him to strive for the extreme control he desires in the series.
Secondly, introduce other characters that can act as catalysts for Overhaul's character development. It could be a hero who challenges his beliefs, or another villain who has a different view on things. Their interactions can bring out different sides of Overhaul. For instance, if he meets a hero who has a very loose and carefree attitude towards life, it could really grate on Overhaul's sense of order and make him question his own rigidity.
Create unique settings. Don't just stick to the usual locations in the 'Boku no Hero Academia' world. Maybe introduce a hidden underground society where Overhaul has a different role or influence. This will add a sense of novelty to your fanfiction.
Fanfiction Watch Fanfiction could potentially be a platform or community dedicated to observing, discussing, or monitoring fan - created fictional works. It might be a site where fans gather to keep track of new fanfictions in their favorite fandoms, offer reviews, or engage in conversations about the various stories written by other fans.