One idea could be that the NCIS team is called in to investigate some strange magical occurrences in the wizarding world. Maybe they think it's some new form of high - tech weaponry at first but then realize it's magic. Another could be that Harry Potter and some of his friends end up in the NCIS world and have to work with the team to solve a mystery that involves both magic and muggle technology. For example, a dark wizard is using muggle technology to enhance his magic and they need to stop him.
A really cool plot might be that Voldemort has somehow managed to create a connection between the wizarding world and the world of NCIS. His Death Eaters are causing chaos in both worlds. The NCIS team has to learn about magic quickly to fight against the dark forces, while Harry and his friends have to adapt to the muggle world's ways of investigation. They could combine their skills, like the NCIS' forensic knowledge and the magic of the wizards, to track down Voldemort's plans and stop him before he takes over both worlds.
One plot idea could be a joint case where the NCIS team and the NCIS LA team have to work together to solve a crime that spans across different locations. Maybe a terrorist threat that has connections in both Washington D.C. (where NCIS is based) and Los Angeles.
Another plot could involve a crossover of villains. A dark wizard from the Harry Potter universe teams up with a criminal that the NCIS has been chasing. Harry Potter and the NCIS agents then have to join forces to stop this unholy alliance. They would need to combine their skills, with the wizards using magic and the agents using their detective and combat skills.
The idea of a dark wizard being involved in a crime that NCIS is investigating is really engaging. It forces the two groups to collaborate. Also, the way some fanfictions handle the different forms of evidence is cool. In the Harry Potter world, evidence can be things like potion ingredients or spells, while in NCIS it's more about fingerprints and forensics. When these two types of evidence - gathering merge, it creates unique plot twists.
One idea could be that the NCIS team is investigating a series of strange disappearances that turn out to be related to the experiments that created the Maximum Ride characters. The team could cross paths with the flock while trying to uncover the truth behind a shadowy organization involved in genetic manipulation.
One idea could be that a case in NCIS leads them to discover a secret project related to the genetic experiments in Dark Angel. Maybe they find a link between a murdered Navy officer and a group trying to recreate the transgenic experiments. So, the NCIS team has to work with Max from Dark Angel, who has her own knowledge and skills in this area.
A plot could revolve around a character from one show being framed for a crime, and the other team has to step in to prove their innocence. For example, a member of the Five - 0 team is wrongly accused of stealing military secrets, and the NCIS Hawaii team uses their resources to dig deeper and find the real culprit. This could lead to some great character interactions as they learn to work together.
One idea could be that a case in Boston (Rizzoli and Isles' territory) has connections to a military - related case that NCIS is investigating. For example, a murder victim in Boston turns out to be a former Navy officer, and both teams have to work together to figure out the motive and the killer. They might clash at first due to different investigation styles, but then gradually learn from each other.
Well, there could be a story where a powerful dark magic object from the wizarding world is brought to Sunnydale. Buffy discovers it and realizes it has the power to disrupt the balance between good and evil in her world. She then travels to Hogwarts to seek help from Harry Potter. Along the way, she meets various magical creatures and has to navigate the wizarding world's politics. Once she meets Harry, they discover that there is a common enemy behind all this, someone who wants to use the chaos in both worlds for their own gain, and they must work together to stop this mastermind.
One idea could be a time - travel plot where a character from Legacies goes back in time to the era of Harry Potter. They could interact with the young Harry and his friends, and perhaps help in the fight against Voldemort in unexpected ways. For example, they might introduce a new spell from the future that gives the Order of the Phoenix an edge.
One idea could be that the Grimm creatures start to appear in the wizarding world. Harry and his friends have to use both magic and knowledge of Grimm lore to fight them. For example, they might discover that some dark magic has opened a portal between the two universes.