Well, in first encounter Sonic fan fiction, I think Shadow would be a great character to include. He has a complex past and his first encounter with Sonic could lead to some intense and interesting storylines. Amy Rose can also be included. Her love for Sonic and her spunky personality can add a different kind of dynamic to the story. Big the Cat could be an interesting choice too. His laid - back nature and his fishing hobby could provide some comic relief in a story about Sonic's first encounters. And Rouge the Bat, with her thieving ways and her connections to different groups, could bring in some intrigue.
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.
One of the most popular themes is adventure. Sonic is known for his speed and his adventures in the Sonic games, so it's natural that fan fiction often features him going on new and exciting adventures. Another common theme is friendship. Sonic has a group of friends like Tails and Knuckles, and stories about their relationships and how they support each other are quite popular. Also, the battle against evil is a big theme. Sonic often has to face off against villains like Dr. Eggman, and fan fictions expand on these battles, sometimes introducing new villains or more complex plotlines for the battles.
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.
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.
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.
Shadow would be a great addition. His brooding nature and powerful combat abilities would add an interesting dynamic. He could be suspicious of Infinite at first and be in conflict with Sonic about whether to trust him or not. And then there's Knuckles, who could guard some important location or object that is related to the crossover event, like a powerful chaos emerald that is causing the dimensions to cross.
Obviously, Infinite himself is a must - include. His complex character with his powers and his connection to the Phantom Ruby can drive a lot of the story. Sonic is also essential as he is the main hero and his interactions with Infinite can be really interesting. Tails could also be included as he can provide the technical support and his loyalty to Sonic adds another layer to the story.
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.
Stone Cold Steve Austin would be a great addition. His rivalry with The Rock was iconic, and adding him to a story with Trish Stratus could create some interesting dynamics. Maybe he could be a reluctant ally or an unexpected adversary.
Obviously, Supergirl herself should be included. Her presence can add more drama and conflict, especially if she has mixed feelings about Lena and Mon - El's relationship. J'onn J'onzz could also be a great addition as he can offer his wisdom and alien perspective.