Well, you could write a fanfic where the main character gets separated from the Fourth Wing and has to survive on their own in the dangerous world outside. They could meet new characters, form new alliances, and develop new skills. Another idea is to write about a forbidden romance between two characters from different wings. This would add a lot of drama and tension. And finally, you could write a fanfic that is set in the future of the Fourth Wing world, where new threats have emerged and the characters have to adapt to new situations.