Anyone heard of the story? Campione? A light novel series. I'm about to write a fanfiction inside of it. Problem. The worldbuilding is terrible. Ok, short story. Protag kills a god, and usurps his powers. As expected, this makes you overpowered, letting him easily defeat 99% of the rest of the setting. The other 1%, are demigods, gods, fellow devil kings, and nothing else. He attracts people to him. Prodigies. Immensely powerful people. People with rare and useful skills. Spies and agents from other organizations seeking to manipulate him. Its a fun world, with a nice mythology and creation of antagonists. The problem is, it just doesn't flesh out anything on the ground level. I know next to nothing about ground level mages. Nothing about how the average mage lives. I barely know the magic system.* I know there's a mafia, and some knight organizations, and various other organizations meant to keep the masquerade. But nothing much. So... anything to help? *Chants and spells seem to take the form of beseeching or asking powers from the gods to shape the effect, and using their own mana to power it. Strongest can smash apart cities, but those are really rare. They will be defeated by anything mildly divine, though.
Friend, so many people have viewed this thread and not replied because we have no idea what that book series is. BUT, on the subject of fan-fiction, I would recommend just not bothering. If the world-building is garbage and you can see that, it means you have in mind a better world with better characters and better plot ideas. My advice is by all means use this book series as inspiration, but write your own story with all-new everything! Believe me, you'll feel better for it and you'll run into issues way less when you can literally just change a whole set of ideas for new ones without ruining the story. Oh and trust me, enough stories use demi-gods, demons, devil kings and things like that. As long as you don't rip-off that series in a really obvious way, you'll have a much better time writing something new. Go for it bro! Happy writing!
Well, the most transformative authors in my life have been Garth Nix (Keys to the Kingdom), Stephen Hunt (Court of the Air) and the authors of the Deepwoods books, Chris Riddell and Paul Stewart. I enjoy fantasy aimed at a younger audience because it's often far more entertaining. Stephen Hunt is a great bridge between that and more hardcore fantasy authors like Brandon Sanderson and Geroge R. R. Martin.