One idea could be a mystery where Natalie goes missing and Monk has to use his unique detective skills to find her. He has to overcome his own OCD tendencies while dealing with the chaos of the situation.
One way is by having them face personal challenges together. Maybe Natalie has a family issue and Monk, despite his own problems, steps up to support her. This could deepen their bond as friends and colleagues. It could also lead to a more emotional connection as they open up to each other about their pasts and fears.
One common theme is the growth of their relationship. Fanfics often explore how Monk and Natalie become closer over time as they solve cases together. Another theme is Monk's struggle with his OCD and how Natalie helps him through it.
There's also 'Monk and Natalie: A New Adventure'. In this fanfic, they find themselves in a small town solving a mystery that seems to be connected to Monk's past. The story is full of the typical Monk - like deductions and Natalie's support for him. It shows how their bond has grown over time.
One idea could be a story where Adrian Brody's character is a lone survivor in a post - Predator - attack world. He uses his wits and skills to outsmart the Predators that are still lurking. Maybe he teams up with other survivors who initially don't trust him but soon realize his value in the fight against the alien hunters.
A really interesting plot could be set in an alternate universe where the wizarding world has different social hierarchies. Hermione, being a Muggle - born, has to face more challenges, and Adrian Pucey, from a pure - blood family, has his own set of expectations. Their relationship develops as they both try to break free from the constraints of this new world order. For example, they might team up to fight against a discriminatory law that targets Muggle - borns. This would involve a lot of character development and exploration of the wizarding world's politics.
One common theme is mystery - solving. Since Monk is a detective, many fanfictions build on his skills to solve new and complex cases.
Perhaps Nick and Natalie are trying to start a new vampire - human coexistence initiative. As adults, they are more aware of the importance of peace between the two species. They face a lot of opposition from both sides. Some humans are afraid of vampires and some vampires think humans are inferior. Nick and Natalie have to use their relationship as an example of how vampires and humans can work together. They hold meetings, give speeches, and try to convince everyone that coexistence is possible.
One idea could be Naruto using his ninja skills in a more spiritual way as a monk, like using chakra control for meditation purposes. Another could be him spreading the teachings of the monastery through his adventures. Maybe he'd use his shadow clones to help with the monastery's chores and tasks.
One idea could be a fanfic where David and Natalie go on a secret adventure outside of their work duties. They discover a hidden part of the city and have a really personal conversation that deepens their connection.
Another fanfic concept could be a time - travel one. Stiles accidentally sends himself, Adrian, and Harris back in time. They end up in a different era, say the 1920s. Now they have to figure out how to get back while also dealing with the cultural shock and the new rules of that time period. And along the way, they might even change some historical events without realizing it.