Perhaps a plot where the Dark King discovers a hidden power within himself. This power could be the key to saving his kingdom from a long - predicted doom. But to fully master it, he has to go through a series of trials, both physical and mental, that test his limits as a leader and as an individual.
Another plot might involve the Dark King's love interest. He could be in love with a commoner, and this relationship causes a stir in the court as it goes against the traditional norms. There could be many obstacles in their way, like the disapproval of the nobles and the interference of the king's enemies.
One interesting plot could be the Dark King's struggle against an ancient evil that threatens his realm. Maybe he has to gather allies from different corners of his kingdom, some of whom are reluctant at first but later come to see the importance of the fight.
Well, a plot could be that King Sombra is resurrected by some dark magic, but this time he has a different goal. Instead of just ruling Equestria through fear, he wants to find a way to break the cycle of hatred. He starts to interact with the ponies in a strange way, and they are both scared and intrigued. There could be a pony who tries to understand him and change his view of the world.
One interesting plot could be the dark prince's struggle with his inner darkness. Maybe he has to make a choice between following his dark instincts or a path of redemption. For example, he meets a character who shows him kindness and makes him question his ways.
Perhaps a plot where Orion has to face his inner fears represented by the dark. It could involve him having to make difficult choices to overcome the darkness that surrounds him. He might find allies along the way who teach him how to control the power of the dark rather than be consumed by it.
Well, in some 'Hermione and Harry Dark Fanfiction', there might be a plot where they enter a dark, alternate magical realm. In this realm, they have to rely on their friendship and skills in new ways. They could encounter dark versions of their friends and have to fight against them, which would really test their loyalty to each other.
One interesting plot could be Naruto getting trapped in the SAO world and having to use his ninja skills in a virtual setting. For example, his shadow clone jutsu could be used in unique ways to overcome challenges in the game, like sending clones to explore different areas of the virtual world simultaneously.
Another plot could be a twist in the magic system. Suppose one of the sisters discovers a new form of magic that was previously unknown. This new magic could either give her an edge in the competition or force the other sisters to adapt and learn it as well. It could also attract the attention of dark forces or ancient entities that have their own agendas regarding the queenship.
One interesting plot could be the Dark Prince having to face his inner demons while trying to protect Hogwarts. Maybe he discovers a new, dark magic that he must either control or destroy.
One interesting plot could be the exploration of a darker side of the Naruto universe where the main characters face moral dilemmas they never had to in the original series. For example, Naruto might have to make tough choices between saving his friends and sacrificing his own principles in the 'dark empire' setting.
Perhaps there's a prophecy that the others know about and they believe that by keeping Harry in the dark, they can change the outcome. They might think that if Harry knows the details of the prophecy, he'll act in a way that will lead to a bad end, so they're trying to manipulate events from behind the scenes without his knowledge.
One interesting plot could be the exploration of a new, hidden power that the main character in the fanfiction discovers. This power might change the dynamics of the story and lead to new adventures.