One interesting plot idea could be a serial killer who targets both the FBI agents from Criminal Minds and the SWAT team members. The two teams have to combine their skills, with the Criminal Minds team analyzing the killer's psyche and the SWAT team providing on - ground protection and tactical support. For example, the killer might be leaving cryptic messages that only the profilers can decipher, while the SWAT team has to deal with the physical threats the killer poses during their investigations.
A plot could involve a hostage situation where the hostages are somehow related to a case the Criminal Minds team is working on, and the SWAT team is called in. The Criminal Minds agents try to figure out the motives of the hostage - takers through their behavioral analysis, while the SWAT team plans the rescue operation. Maybe the hostage - takers are demanding the release of a criminal that the Criminal Minds team put away, and there's a race against time to resolve the situation without any casualties.
Well, a cool plot might be that Sherlock and Reid from Criminal Minds get into a battle of wits over a case. The case could involve a super - intelligent criminal who is playing mind games with both of them. Or perhaps there's a case where the BAU's profiling skills are needed to understand the motives of a criminal that Sherlock has been chasing, but can't quite figure out the 'why' behind the crimes. And they end up learning from each other's methods.
One idea could be that the Scorpion team is called in to help the Criminal Minds team with a case involving a highly intelligent criminal who is using complex technology to carry out their crimes. For example, the Scorpion team could use their skills to decrypt the criminal's encrypted messages that the Criminal Minds team couldn't crack on their own.
One idea could be that the BAU team from Criminal Minds is called in to investigate a series of murders that seem to have a connection to Dexter's 'hobby'. Dexter, being the secret serial killer who targets other killers, might initially be a suspect. Then, as the story unfolds, they could end up working together to take down a bigger network of criminals that neither could handle alone.
A crossover with 'Sherlock' would be cool. Imagine Sherlock's deductive reasoning combined with the BAU's psychological profiling. They could be working on a case that involves a criminal mastermind who is playing mind games with both teams. Sherlock might initially be dismissive of the BAU's methods but then come to respect them as the case progresses.
One idea could be that the BAU team is called in to investigate a series of strange disappearances that seem to be related to a haunted location that Teamiplier has explored in one of their videos. The two groups initially clash due to their different approaches, but then they start to work together and combine their skills to solve the mystery.
One idea could be that the BAU is called in to help the NCIS LA team when a series of murders seem to be related to a military - civilian connection. The agents from both teams could clash at first due to different working styles, but then gradually learn from each other and solve the case together.
One idea could be that the Scooby Gang from Buffy teams up with the BAU from Criminal Minds to solve a series of murders that seem to have a supernatural and a psychological aspect. Maybe the murders are being committed by a demon that is using human psychological weaknesses to target its victims.
Maybe in the fanfiction, a member of the CSI team has a personal connection to a case that the Criminal Minds team is investigating. This could lead to some intense emotional moments and also some friction between the two teams as they try to balance the need for objectivity with the personal investment. The story could explore how they overcome these challenges and solve the mystery behind the crimes.
The serious and cerebral profilers from Criminal Minds might initially clash with the more action - oriented SWAT team members. For example, Reid from Criminal Minds might seem overly intellectual and reserved to some of the SWAT guys who are used to more straightforward communication. But over time, they could learn from each other. The SWAT team could teach the Criminal Minds agents about physical combat and quick - response tactics, while the agents could help the SWAT team understand the psychology behind criminals they face.
Well, you could write a fanfiction where the BAU team gets trapped in a virtual reality world created by a tech - savvy unsub. They have to solve puzzles and cases within this virtual world in order to escape. Or perhaps a story where a new agent joins the team who has a psychic ability that they keep hidden at first. But as the cases progress, this ability starts to surface and help the team in unexpected ways.