Knuckles can be a really interesting character to include. His strength and his role as the guardian of the Master Emerald can bring in elements of mystery and ancient lore. Amy Rose can also add a touch of romance and determination to the fanfic with her unwavering love for Sonic.
Sonic himself is a must - include. His speed and adventurous spirit drive the story forward. Tails is also great as he can provide technical support and is Sonic's loyal friend. And Dr. Eggman, as the main antagonist, always adds conflict and excitement to the story.
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.
Tails could be a great addition. He could be Sonic's sidekick who initially doubts Sonic's new relationship with Dash but later comes to accept it and even helps them out in some tricky situations. Another good character could be Rainbow Dash's friend Fluttershy. She could offer some gentle advice to Dash about love and relationships.
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.
Mario and Sonic themselves are obvious choices. They are the main characters of their respective series and their rivalry - turned - friendship can be a great focus. Luigi is also popular as Mario's brother, and Tails is a fan - favorite in the Sonic series. Tails' intelligence and his ability to fly can add an interesting dynamic to the story.
Vegeta is a great choice. His rivalry with Goku adds a lot of drama and action to a fanfic. Also, Piccolo, as he has a long history with Goku and has been both an enemy and a friend. Another good character could be Gohan, Goku's son, as it can explore their father - son relationship further in the fanfic.
Another good character to include is Lex Luthor. He could be the mastermind behind a plot to drive a wedge between Batman and Wonder Woman. His devious mind and resources would make him a formidable antagonist, and his interactions with the two heroes would add a lot of drama to the story.
Optimus Prime is a great choice. His leadership and wisdom can add depth to the story, and his relationship with Bumblebee can be explored further. Also, Sam Witwicky, as he has a strong bond with Bumblebee in the movies. Their friendship can be a central part of the fanfic.
Definitely Naruto himself, as he is the main character of the original series. Sasuke is also a great choice as his complex relationship with Naruto can add a lot of drama. Kakashi is another good character to include. His wisdom and unique abilities can be very useful in the story. And Sakura, her growth as a ninja can be explored further in a fanfic.
Well, obviously Armikasa themselves are the main focus. But you could also include their friends. For example, their comrades who are with them in training or on missions. These characters can add more depth to the story by providing different perspectives and interactions.