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.
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.
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.
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.
I really like 'boku no hero academia shouko fanfic'. The character Shouko is often portrayed in interesting ways in these fanfics. Some fanfics focus on her relationships with other characters from the series, which can add new depth to the overall story. For example, her interactions with the main protagonist can be really heart - warming or full of tension, depending on the writer's take.