Well, he could be involved in a political intrigue within the kingdom. Maybe he's framed for a crime he didn't commit and has to clear his name. Or, he could form an unlikely alliance with a group of outcasts, like thieves or rebels, and together they try to change the unjust system of the kingdom. Also, a love story where he falls in love with a commoner and has to fight against the class differences and the rules of the royalty to be with her.
A possible plot for a 'bastard prince fanfic' is that the prince discovers a secret prophecy that involves him. This prophecy could be the key to saving the kingdom from an impending doom, but the other royals don't believe in him. So, he has to go on a quest to find the elements needed to fulfill the prophecy. Along the way, he meets different characters who either help or hinder him. He also has to deal with his own insecurities about being a bastard and whether he's truly worthy of such a great task.
Another plot idea is to create an alternate universe where Jude and Cardan are forced to work together from the very beginning to stop a greater threat to Faerie. This would change the dynamics of their relationship and the overall story.
One idea could be Percy having to deal with a new, powerful enemy that even the Olympians are afraid of. He has to use his unique skills and form new alliances.
Well, a plot idea could be that Cinderella and Prince Charming have a big fight right after their wedding. Maybe Cinderella feels suffocated by the royal life and Prince Charming doesn't understand her at first. But then they go on an adventure together outside the kingdom and rediscover their love. Another idea is that there's a curse on Prince Charming that makes him turn into a beast at night. Cinderella has to find a way to break the curse while still loving him in his beastly form. Also, we could have a story where Cinderella is kidnapped by an evil witch who wants Prince Charming for herself. Prince Charming has to rescue Cinderella and prove his love is true.
One idea could be that Cersei's bastard rises to power in secret. He or she could form alliances with unlikely characters, like some of the smallfolk who have been overlooked. They might use their wits and charm to gain support, all while hiding their true identity from the powerful families in Westeros.
One idea could be Percy having to face a new underwater threat that is specifically targeting Atlantis. Maybe a powerful sea monster that was once imprisoned by the Atlanteans has been released and Percy has to figure out a way to stop it without causing too much destruction to the underwater city.
Well, you could write a fanfic about the adventures of the dragon prince himself before he met Callum and Rayla. Maybe he had some encounters with other magical creatures that shaped his view of the world. Or you could focus on a post - series fanfic where the kingdoms have finally achieved peace, but new, unexpected threats emerge from the shadows. Also, a fanfic about Ezran's growth as a king, facing difficult decisions that test his morality, would be quite interesting.
One interesting aspect could be the unique character perspectives it might offer. Since it's a fanfic, it may explore the story from the point of view of a bastard in Westeros in a way that the original series didn't fully. It could also introduce new plotlines and relationships within the established Westeros universe.
One interesting plot could be about the characters facing a new business challenge together and how their relationship evolves during it. Maybe they have to work on a project in a foreign country and deal with cultural differences while also dealing with their own feelings for each other.
Well, a good plot could be about a competition. There's a grand competition announced in the Candy Kingdom, and both Prince Gumball and Princess Bubblegum enter it. They start off as rivals, each determined to win. But as the competition progresses, they realize that they can achieve more by working together. So they form an alliance and end up not only winning the competition but also strengthening their bond as friends or more.
Maybe the Black Prince discovers a new form of Geass power that no one else has ever seen. He then has to figure out how to control it while also hiding it from those who would seek to use it for their own evil purposes. This could involve a journey of self - discovery as he battles his inner demons as well.