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.
You could look on well-known online movie libraries such as Hulu or HBO Max. Also, some international movie streaming services might have it. Just make sure to have a valid subscription and a stable internet connection.
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.
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.