A great concept would be a crossover where a member of the NCIS team has a family curse that the Winchesters are familiar with. The NCIS agent is skeptical at first, but as the evidence mounts, they have no choice but to work with Sam and Dean. Along the way, there are some really interesting character interactions, like Gibbs trying to understand the Winchester brothers' chaotic lifestyle and Dean being impressed by Gibbs' leadership skills.
One idea could be that the NCIS team is investigating a series of strange murders that seem to have a supernatural element. They end up calling in the Winchesters from Supernatural for help. The two groups initially clash due to their different methods but eventually learn from each other and solve the case together.
Maybe there's a case where an ancient artifact that the NCIS comes across has some sort of demon or other supernatural entity trapped in it. This draws the attention of Sam and Dean Winchester. As they work together, the NCIS agents start to believe in the supernatural world more and more, and the Winchesters get to see how a real - world investigation unit operates.
One idea could be that some of the NCIS agents are sent on a mission to a world that has similarities to the Naruto universe. They could team up with Naruto and his friends to solve a mystery that involves both their worlds, like a powerful weapon that could disrupt both dimensions. The NCIS agents would be shocked by the ninja skills and chakra usage, while Naruto and his friends would be intrigued by the advanced technology of the NCIS.
One great Supernatural and NCIS crossover fanfiction is 'The Hunt in Quantico'. It combines the mystery-solving aspect of NCIS with the supernatural elements of Supernatural really well. The characters from both shows interact in a very believable way, and the plot keeps you on the edge of your seat.
When writing a Supernatural and NCIS crossover fanfiction, it's important to create a believable setting. You can start by setting the scene in a location that both shows could potentially use, like a military base with a haunted history. Also, think about how the different mythologies in Supernatural, like demons and angels, would be perceived by the more scientific - minded NCIS agents. You'll need to build up the relationship between the characters gradually. Maybe start with some suspicion and then develop into mutual respect as they work together to solve the mystery.
One idea could be that the teams are brought together to solve a case involving a military - related crime that has ties to Hawaii. Maybe a stolen military weapon is being sold on the black market there. The characters from Hawaii Five - 0 can provide local knowledge and connections, while the NCIS agents bring their expertise in military investigations. Another idea is that a Navy officer goes missing in Hawaii and both teams have to work together to find him. They might initially have some clashes in their working styles but eventually learn from each other.
One idea could be that a case in Hawaii has connections to a national security threat that NCIS is already investigating. For example, a suspected terrorist cell in Hawaii might be linked to a larger international network that NCIS has been tracking. Agents from both teams could collaborate, with the Hawaii Five 0 team providing local knowledge and the NCIS agents bringing in their expertise on international threats and military - related matters.
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.
One idea could be that a case starts in Hawaii and has connections to Los Angeles. The teams from both shows have to collaborate. Maybe a criminal organization is operating in both locations and they are smuggling some high - tech weapons. The characters from Hawaii Five 0 like Steve McGarrett could team up with the NCIS LA agents like G. Callen to take down the bad guys.
Perhaps a fanfiction where a member of the Criminal Minds team, say Reid, gets framed for a crime that involves national security, which brings in the NCIS. The NCIS team initially suspects Reid but as they dig deeper, they realize he's been set up. Together, they work to clear Reid's name and find the real perpetrator who is using the situation to cover up a much larger conspiracy that spans both military and civilian sectors.
Maybe there's a situation where an alien artifact is found on Earth that has a connection to Atlantis. The NCIS team starts investigating it, not fully understanding what it is at first. Then the SGA team gets involved because they recognize the technology. They work together to keep the artifact safe from various threats, both Earth - based and alien.
Maybe there's a hidden Templar plot that has ties to a military case the NCIS is investigating. An Assassin discovers this connection and has to work with the NCIS agents, who are initially skeptical of this person with such strange abilities and knowledge. They gradually learn to trust each other as they uncover the Templar conspiracy together.