Obviously Danny and Rouge are the main ones. But Tucker could be a great addition. He could provide some tech - support or comic relief. And for Rouge's side, maybe Shadow could be included. Shadow could add an element of mystery and power to the story.
In addition to Danny and Rouge, Jazz could be a really interesting character to include. Jazz is Danny's sister and she's very intelligent and perceptive. She could help Rouge understand Danny better or vice - versa. From Rouge's world, Omega could be included. Omega's brute strength and loyalty could create some interesting dynamics, especially if he has to interact with Danny's ghostly powers or Tucker's gadgets.
Danny himself is a must - include. His powers and his dual identity would add a lot of interesting elements. Tucker and Sam are also great choices as they could be Danny's support system while dealing with baby - related situations. Jazz could be included as she often has a more mature and understanding perspective. And of course, Danny's parents, who might be clueless about what's really going on with the baby but still add a lot of humor.
Definitely Sam and Tucker. They are Danny's best friends in the Danny Phantom series. Including them in the fanfiction can add some humor and also show Danny's normal life side. They could also interact with Ophelia in interesting ways, like Sam being a bit jealous at first of Danny's closeness to Ophelia.
Cisco Ramon from the Flash's universe would be a great addition. His ability to create new gadgets and his sense of humor would fit well in a crossover. For Danny Phantom's side, Frostbite could be included. His knowledge of the Ghost Zone and his ice - based powers could be crucial in a story where they are facing a powerful enemy. Also, including the Reverse Flash as a villain would create a lot of tension and action in the fanfiction.
Sam is a great character to include. She often provides a different perspective to Danny and Tucker's adventures, and her environmentalist views can add an interesting dynamic, especially when dealing with ghosts that are related to nature or pollution.
Obviously, Danny Phantom himself is a must - he's the main character. Then, the Control Freaks are essential as they are the focus of this particular fanfiction. Tucker and Sam are also great to include as Danny's friends. They can provide support, different perspectives, and sometimes comedic relief. Jazz could also be added, maybe she gets caught up in the Control Freaks' schemes in a way that forces Danny to act to protect her.
Sam Manson would be an excellent addition. Her strong - willed and goth - like nature can add an interesting dynamic. From the Kamen Rider side, if we consider Kamen Rider W, Shotaro Hidari could be included. His detective skills combined with Danny's ghost - hunting abilities would make for some great mystery - solving scenes. And let's not forget Jazz Fenton, Danny's sister. She could provide some emotional support and also get involved in the action in her own way.
Tucker would be a great addition. He could provide some comic relief with his tech - savvy ways. He might also be able to upgrade their ghost - fighting gear. Sam is another good one. She could offer her unique perspective on things, especially when it comes to environmental and ethical issues. And of course, Vlad could be included as the antagonist. His devious plans and power - hungry nature always make for an interesting conflict.
Sam is also quite popular. Her goth - like personality and her strong - willed nature make her an interesting character. In fanfiction, she can be portrayed in different ways, like being even more involved in Danny's ghost - fighting activities.
I think Jazz could be included. She could be the voice of reason for Danny, helping him understand his feelings for Ember. The Box Ghost could also be in the story for some comedic relief. And Technus. He might try to use the relationship between Danny and Ember for his own evil technological plans, which would add an interesting twist to the fanfiction.
In Danny Phantom experiment fanfiction, Danny's parents, Jack and Maddie Fenton, are obvious choices. Their knowledge of the supernatural and their inventions could play a big role in the experiment. Then there's Valerie Gray. She could be another test subject in the same experiment as Danny, which would create an interesting dynamic between them. Dash Baxter could also be included in a unique way. Maybe he accidentally stumbles upon the experiment and his reaction to seeing Danny's powers could add some humor. And let's not forget Clockwork. He could be the one who foresaw the experiment going wrong and tries to guide Danny through the chaos.