The Grim Reaper from 'The Grim Adventures of Billy & Mandy' could be an interesting addition. He could bring some dark humor and a different kind of magic to Dexter's world of science.
One idea could be Dexter crossing over with another smart character like Jimmy Neutron. They could compete in a science fair, leading to all kinds of wacky inventions and mishaps.
One possible plot could be that one of Dexter's female friends or a female character in the Dexter Laboratory world discovers she is pregnant. She might then turn to Dexter for help, thinking his scientific genius could assist in some way, like creating a device to monitor the baby's health in a unique way.
As I mentioned before, Dee Dee could be a likely candidate. She's always getting into Dexter's business, and a pregnancy could add an interesting layer to her character. It could show a more serious and responsible side of her as she prepares to be a mother.
It could be Dexter's teacher. If she got pregnant, it would add an interesting layer to the school environment in the Dexter Laboratory. The students, including Dexter, might be curious and try to understand what's going on. And she could be dealing with all the normal pregnancy stuff while also trying to teach her students, which could lead to some comical and heartwarming situations.
One idea could be that one of Dexter's female relatives, like his sister Dee Dee, accidentally gets pregnant. This could lead to chaos in the laboratory as Dexter tries to figure out how to deal with the situation while also keeping his scientific experiments running. Maybe he creates some sort of device to help with her pregnancy symptoms or to keep her out of his lab during her mood swings.
The plot should be engaging. A good crossover fanfic will have a plot that combines the elements of both universes in a seamless way. For instance, if it's Dexter crossing over with a fantasy world, the mystery or crime aspect of Dexter's world should blend well with the magic or other elements of the fantasy world.
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.
One great Dexter crossover fanfic is 'Dexter Meets Sherlock'. It combines the dark, analytical mind of Dexter with Sherlock's brilliant deductive skills. The story is full of suspense as they both try to solve a complex serial killer case that has elements related to both their worlds.
Hotch from Criminal Minds could be very interesting. He's a strong leader and his no - nonsense approach would contrast with Dexter's more secretive and morally ambiguous ways. Their interactions could be intense, like when Hotch starts to suspect that Dexter is not what he seems. Hotch would try to use his profiling skills to figure out Dexter, while Dexter would be trying to outwit him to protect his secret.