It starts at a specific point in the manga, but I can't recall exactly without looking it up. You might want to check some manga forums or dedicated fan sites for the precise chapter.
It's hard to say for sure. Both Sukuna and Gojo are incredibly powerful, and it could depend on various factors like their current state and the circumstances of the fight.
Sukuna does die in the manga. The way it happens is quite dramatic and ties in with many other elements of the story, adding depth and tension to the overall narrative.
The Harry Potter fandom is great for oc fanfiction. There are so many aspects of the wizarding world to explore, like creating a new student at Hogwarts with unique magical abilities. Another good one is the Star Wars fandom. You can create original characters within the vast galaxy, perhaps a new Jedi or a smuggler in a far - off system. The Marvel Cinematic Universe is also a top choice. You could develop an original superhero or villain with powers related to the existing lore.
One of the top fandoms for fanfiction is the Doctor Who fandom. With its time - traveling and space - exploring premise, the possibilities are endless. You can create new adventures for the Doctor and his companions, or introduce new alien species. The Supernatural fandom is also very popular. There are so many different mythologies and creatures in the show that you can base your fanfiction on. And then there's the Percy Jackson fandom. It combines Greek mythology with modern - day settings. You can write about new quests for demigods, or explore the relationships between different characters from different cabins at Camp Half - Blood.