Perhaps a fanfiction could focus on the backstory of Harvey Dent before he became Two - Face. It could show his early days as a prosecutor and how his idealism gradually got corrupted, which would add more depth to his character in the Dark Knight universe.
One idea could be exploring what if Batman had a different sidekick in the 'the dark knight rises' fanfiction. Maybe a young girl with special tech skills who helps him in new ways.
One interesting plot idea could be exploring Bane's backstory more in - depth. Maybe delving into his early days in the prison and how he became the menacing figure we see in the movie. Another could be a different take on his relationship with Talia al Ghul, like if there were hidden motives or unspoken feelings between them.
One idea could be a story where Rachel survives and she and Bruce try to have a normal relationship while still dealing with the crime in Gotham. Maybe she becomes more involved in his crime - fighting efforts in a non - violent way, like helping with the rehabilitation of criminals he catches.
One plot idea could be Harley trying to break free from the Joker's control in a more psychological way. Maybe she starts having dreams that show her a different life without him, and she begins to question their relationship while still being in love with him. Another idea is a story where the Joker and Harley team up with other villains from the Dark Knight universe against a common enemy. For example, they could join forces with Bane to take over Gotham, but there are internal power struggles within their alliance.
Perhaps a fanfiction could explore what would happen if the Dark Knight was suddenly the only one with his memories intact in the Justice League due to some sort of mind - wiping event. He then has to slowly regain the trust of his teammates while also trying to figure out what caused the memory loss and stop a new threat that is emerging, all while operating in the shadows as he usually does.
One plot idea could be that Jane somehow gets caught in the middle of the battle between Batman and the Joker. Maybe she starts off as an ordinary citizen who witnesses a crime related to the Joker's chaos. As she gets involved, she discovers that the Joker has a strange fascination with her, seeing her as a new kind of plaything. This could lead to a series of tense encounters where Batman tries to protect her from the Joker's unpredictable and dangerous schemes.
A plot idea might be that a long - lost relative of Ra's al Ghul comes to Gotham seeking revenge for his death. This new character could bring in some of the League of Shadows' old ways but also modern twists. Also, we could see the return of Catwoman in a more complex role, perhaps having to choose between her own interests and the good of Gotham.
One idea could be a crossover where the Jedi from Star Wars encounter the moral dilemmas similar to those in The Dark Knight. For example, a Jedi might face the choice of using a dark side power to save many, like Batman's tough decisions. Another could be a Sith lord with the Joker's chaotic nature, causing havoc in the Star Wars universe. Maybe a young padawan is influenced by this Sith - Joker figure and has to struggle between the light and dark within himself, just as characters in The Dark Knight trilogy often struggle with their own morality.
A more action - packed plot could be the red knight leading a rebellion against the vampire hierarchy. They gather other vampires and humans who are tired of the oppression and fight for a new order. Along the way, they discover secrets about the vampire world that could change everything.
One idea could be a new character being introduced into the Sonic Black Knight world. This character could be from another dimension and have unique powers. They could initially be seen as a threat but end up teaming with Sonic to fight a greater evil. Another plot could be Sonic losing his sword skills and having to go through a series of challenges to regain them, all while facing off against old enemies with new twists.