Kakashi could also be included. He has a history with Obito, and his relationship with Itachi was also complex. His skills and his role as a mentor - like figure can bring a different dynamic to the story. For example, he could be the one who suspects something strange is going on between Itachi and Obito and tries to figure out their motives.
One idea could be a story where Itachi and Obito are forced to team up against a new, powerful enemy that threatens the ninja world. They start off distrusting each other due to their past actions, but as they face challenges together, they gradually come to understand and respect one another.
An old - time medicine man could also be included. He could either be a healer who tries to cure Doc Holliday's illness or a trickster who causes more problems for him. This would introduce elements of Native American culture and mysticism to the fan fiction.
Obviously Beavis and Butthead themselves are essential. Their unique personalities and idiocy drive most of the stories. Also, Mr. Van Driessen can be a great addition. His attempts to educate them often lead to hilarious situations.
Patrick could be a great addition. His simple - mindedness could contrast with Sandy and Squidward's more complex personalities. He might accidentally cause chaos while Sandy and Squidward are trying to do something important, like a science experiment or a music recital.
Minato might also be a prominent character. As their teacher, he can influence the way Obito and Kakashi interact. In some fan fictions, he could be the one to help them resolve their differences or provide some crucial information about their past that changes their current situation.
Sonic himself is a must - include character. After all, it's Sonic fan fiction. His speed, attitude, and sense of justice are central to any story. Tails is also a great addition. His intelligence and loyalty to Sonic can add depth to the story. And Knuckles, with his strength and his connection to the ancient ruins in the Sonic universe, can bring in an element of mystery. Dr. Eggman is another important character. His evil plans and his constant battles with Sonic are classic elements that can make the fan fiction exciting.
Spongebob is essential as the main character. Gary could be included, and perhaps in the fan fiction, Gary's meows are what lull Spongebob to sleep. Sandy could also be part of it, like she invents a new type of sleeping apparatus for Spongebob. And Mermaid Man and Barnacle Boy could make an appearance in Spongebob's dreams, where he teams up with them to fight off some sleep - stealing villains.
Toad could be a great addition. He's always been Princess Peach's loyal servant. He could try to rescue her in his own bumbling way when Bowser kidnaps her, adding some humor to the story.
Yasha would be a great addition. Her relationship with Beau and Jester could add an interesting dynamic. She could be the muscle when Beau and Jester get into trouble on their adventures.
Well, obviously Shark Boy and Lava Girl themselves are essential. But adding Max, the main character from the movie who brought them to life in his dreams, can add an interesting dynamic. He could be in a situation where he has to call upon them again for a new adventure.