One interesting idea could be the unique powers the man made of lasers might possess. For example, he could have the ability to refract light in ways that create illusions or even manipulate energy on a different level compared to normal characters. Maybe he can use his laser - like body to communicate through light signals in a secret code.
Obviously, the man made of lasers is one of the main characters. He would be at the center of the story, driving the plot with his actions and abilities.
One idea could be a fanfiction where Denji gets transported to a modern-day city outside of his own world. He has to adapt to new technology and deal with a different kind of devil threat. Another could be a story that focuses on Power's past before she met Denji, exploring her time in Hell and how she became the way she is. A third idea might be a fanfiction where Makima is actually a double agent, secretly working against the devils she seems to be controlling.
One idea could be exploring Tony Stark's new inventions after the events of previous movies. Maybe he creates a new type of suit that can travel through dimensions.
You could also create a fanfiction where the frozen criminals start to form an alliance within the prison. They come up with a plan to escape and take over the new 'utopian' society, and the story could be about how the hero stops them.
In Iron Man 3 fanfiction, a common theme is the expansion of the Iron Legion. Some stories might have the Iron Legion being used for civilian rescue operations on a large scale. There could also be fanfictions that explore the villains in more detail. For example, exploring Aldrich Killian's backstory more and what led him to become the way he was, and how his relationship with Tony could have been different.
Another idea could be a story where Spider - Man has to help Ben 10 when the villains from his world start causing trouble in New York. They would have to combine their different skills and knowledge to protect the city and stop the bad guys.
Well, you could write a fanfiction about a new hero who joins the Hero Association and has a unique power that somehow challenges Saitama's world view. For example, a hero who can control time in a very limited way but uses it in the most strategic manner. Or perhaps a story where all the villains team up against the heroes and Saitama has to face an army of powerful foes all at once. It would be interesting to see how he handles that without just one - punching everyone immediately.
One idea could be exploring a different origin story for Spider - Man in the fanfiction. Maybe he gets his powers from a scientific experiment gone wrong in a completely new way. For example, instead of being bitten by a radioactive spider, he could be exposed to a mutagenic serum that gives him spider - like abilities along with some unique side - effects that he has to deal with.
Maybe in the fanfiction, Slender Man has an alliance with Orochimaru. Orochimaru is always interested in strange and powerful beings. Together, they plan to take over the ninja world. Naruto and his friends then have to not only face Orochimaru's usual schemes but also deal with the terrifying abilities of Slender Man.
One idea could be that the main character, who is a betting man, makes a bet with a vampire about being able to survive in the vampire world for a certain period without getting turned. He uses his wits and knowledge of the Twilight universe to outsmart the vampires at every turn.