One interesting Mineta fanfiction could be about him overcoming his more...controversial traits and becoming a more respected hero. Maybe he has a moment of self - realization and starts to focus more on using his quirk for good in a more heroic way.
Websites like Archive of Our Own (AO3) are great places to start. It has a large collection of fanfiction, including many related to 'Boku no Hero Academia' and Mineta. You can search for specific tags related to Mineta to find the ones that interest you.
The Boku no Hero manga is finished. It reached its conclusion after a successful run, leaving fans with a complete narrative and memories of an exciting journey.
Some fanfictions have the theme of Mineta finding his place in the hero world. He is often underestimated, and these stories explore how he can break free from that perception. They might also include themes of loyalty, as he stands by his friends in difficult situations within the My Hero Academia setting.
One popular idea could be creating an alternate origin story for a character. For example, giving Deku a different quirk acquisition experience that still leads to his growth as a hero.
It could be Midoriya. Imagine Midoriya getting access to Iron Man's technology. He's already a determined and intelligent hero - in - training, and with Iron Man's tech, he could become even more powerful. He might struggle at first to adapt to the new technology, but his strong will would drive him to master it and use it to protect others in new and exciting ways.
One great 'Boku no Hero Academia Izuku' fanfic could be one where Izuku discovers a new, hidden quirk within himself that's not related to One For All. This could lead to him having to train in a completely different way and form new relationships with his classmates as they all try to figure out this new aspect of his powers.