Sue Storm is fascinating too. Her invisibility and force - field powers can be used in so many different ways in a story. You could write about how she uses her powers for stealth missions or to protect her team in dangerous situations.
Reed Richards is really interesting. His super - intelligence and ability to stretch his body in incredible ways make for great plot possibilities. You can write about how he uses his brain to solve complex problems and how his stretching powers come in handy in various situations.
The Thing, Ben Grimm, is also a great character for a fanfic. His struggle with his new, rocky appearance and how he deals with being different from his former self can be explored in depth. He also has a tough - guy exterior but a soft heart inside.