Maybe they are running in a magical race. Harry uses his broom - flying skills in parts where he can, but Dudley, being more used to running on the ground, finds shortcuts through Muggle - like alleys that are magically connected. Along the way, they meet various magical creatures that either help or hinder them, and they have to figure out how to deal with them as a team.
One plot idea could be that Dudley somehow gets involved in the wizarding world due to a magical accident. Snape, initially reluctant, has to keep an eye on him. Meanwhile, Harry is caught in the middle, trying to balance his relationship with Dudley and dealing with Snape's grumpiness. For example, Dudley could accidentally drink a potion meant for someone else and start showing magical abilities, leading to chaos and an unlikely alliance between him and Snape to reverse the effects.
A great plot could be centered around Dudley and Harry being forced to work together on a project in the Muggle world that has some unexpected magical implications. This could lead to them understanding each other better and forging a new kind of relationship. For example, they might be involved in an archaeological dig that uncovers a lost magical artifact, and they have to keep it safe from both Muggle and wizarding thieves.
One idea could be that they run away to a hidden magical land where they discover a new form of magic that is neither light nor dark. They have to learn to control it together and in the process, they become closer friends. Another plot could be that they run away because of a false accusation against them in the wizarding world, and on their journey, they find evidence to clear their names.
One idea could be that Ginny and Dudley accidentally meet in a magical mishap. Maybe a spell gone wrong transports Dudley into the wizarding world near Ginny. They initially dislike each other due to their different backgrounds, but as they are forced to work together to get Dudley back home, they start to see each other's good sides.
One possible plot point could be Dudley finally realizing the true nature of the danger Harry is in and using his new - found courage to save him. Maybe he discovers some secret about Voldemort that helps him protect Harry. Another could be Dudley using his physical strength, which he was known for in the original series in a more heroic way to fend off Death Eaters.
One interesting plot idea could be Dudley traveling back in time to when his parents first got married. He could see how his family's attitudes towards magic and the Potters were formed. Another could be him traveling forward in time to see the impact of Harry Potter's actions on the wizarding world and how it indirectly affects the Muggle world he lives in. Maybe he travels to a time when a major wizard - Muggle conflict was about to start and he has to use his new - found knowledge of both worlds to prevent it.
One idea could be a time - travel plot. For example, the Running Man members are sent back to ancient China and have to complete tasks related to historical events to return to their time. They could interact with historical figures and use their modern - day knowledge and skills to solve problems.
An OC who is a master of disguise could be a great plot. They keep changing their appearance during the show, confusing the other players and creating really interesting and comical situations.
One idea could be a time - travel plot. The Running Man characters could be sent back in time to the origin of the game show and have to compete in the very first, more primitive version of the challenges. Another idea is a crossover with other popular anime characters. They could team up or compete against characters from Naruto or One Piece, for example.
Well, a great plot for a Running Man x OC fanfiction could be that the OC is a world - class athlete who comes to the show to prove that they are the best. Along the way, they form friendships and rivalries with the regular Running Man cast. Or perhaps the OC is an alien who crash - lands near the set of Running Man and is forced to participate in the games to earn their way back home. Also, the OC could be a detective who is investigating a mystery related to the Running Man show and has to join the games to gather clues.