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.
In addition to Infinite and Sonic, I think Shadow would be a great character to include. Shadow's own past and his powerful abilities can create some really intense scenes when paired with Infinite. Amy could also be a good addition. Her determination and her feelings for Sonic can bring in an emotional aspect, especially when dealing with the chaos that Infinite might cause.
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.
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.
Meteora herself is of course essential. She has a complex backstory and a lot of potential for character development. Star would also be a great addition as she and Meteora have an interesting dynamic in the show. Their interactions could be explored further in a fanfiction.
Tails is also quite popular. Fans like to write about his inventions and how he uses his intelligence to help Sonic out. There are many fanfictions where Tails comes up with some amazing gadgets to defeat Dr. Eggman.
Sonic, of course, is a must - have. But also Knuckles from both universes. In Sonic Boom, Knuckles is a bit more of a muscle - brained character in a fun way, while the classic Knuckles is more serious about guarding the Master Emerald. When they meet, there could be some hilarious misunderstandings at first, like Sonic Boom Knuckles trying to show off his strength to the other Knuckles, but then they end up teaming up against a common threat.
Sonic himself is always popular. He's the main character of the Sonic franchise, so in crossovers, he often takes center stage. His speed and heroic nature make him a key figure.
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.
Tails is a very popular character to include. He is Sonic's best friend and his technical skills can add a lot to the story. For example, he could help Sonic and Shadow figure out some complex machinery or technology that is part of the plot.
A common theme is the exploration of power dynamics. Since Infinite has his own set of powers and Gadget might have her technological prowess, how they balance or compete in terms of power is often explored.
A prevalent theme is the power of love or connection. In these fanfictions, the relationship between the reader and Infinite can often change Infinite's outlook or actions. Also, self - identity is a theme. Infinite is trying to figure out who he is, and the reader can play a role in that discovery. For instance, in some stories, the reader helps Infinite come to terms with his true self.