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.
Well, 'boku no hero novel' is probably about the adventures in the world of heroes. It could have new storylines that didn't make it into the main anime or manga. It may focus on a particular character's growth, like Midoriya's journey in a different light, or introduce new characters and their unique quirks within the hero - filled universe.
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 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.