Knuckles could also be great. His strength and ability to find hidden things could come in handy. In a crossover with a world that has ancient ruins, like in the Indiana Jones movies, Knuckles could lead the way in exploring the ruins and finding important items or clues that are needed to progress in the story.
Shadow the Hedgehog would add a lot of depth to a crossover fanfiction. His brooding personality and his own set of skills, like his chaos control, could create interesting dynamics. If Sonic crosses over into a darker, more post - apocalyptic world, Shadow could be the one who has already adapted to the harsh environment and could teach Sonic how to survive there.
Tails would be an obvious choice. He could use his mechanical skills to help Sonic in different crossover scenarios. For example, if Sonic crosses over into a world full of advanced technology, Tails could be the one to figure out how the new technology works and use it to their advantage.
I think Knuckles would be a great addition. He could provide some interesting dynamics with Sonic the Werehog. Since Knuckles is strong and has his own set of abilities, he could either be a helpful ally or, in some cases, a bit of a rival in the crossover world. For example, if they were in a world where there was a limited amount of power - like special gems that both of their powers rely on - there could be some tension between them as they try to figure out how to share or compete for those resources.
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.
One idea could be a crossover with another Sega character like Bayonetta. Sonic could team up with her to fight against a common enemy that has the power to both slow down time (like Bayonetta's Witch Time) and manipulate the chaos energy in Sonic's world. They would have to combine their unique abilities to defeat this new threat.
In sonic the hedgehog crossover fanfiction, Sonic himself is of course the main character. But other characters that are popular to include are Amy Rose. Her love for Sonic can create interesting relationship dynamics in crossovers. Another popular one is Dr. Eggman. His villainous nature can be a great source of conflict in crossovers with other characters. And then there are characters from other video game series that are also quite popular, like Link from The Legend of Zelda. His use of weapons and exploration skills could make for some really exciting crossovers with Sonic.
From other franchises, Link from The Legend of Zelda could be an interesting addition. His sword - fighting skills and puzzle - solving abilities would contrast with Sonic's speed - based combat. He could be on a quest in Sonic's world and end up teaming up with Sonic to defeat a common enemy. Another character could be Samus Aran from Metroid. Her powerful weapons and armored suit could bring a new level of badassery to the fanfic, and she could have some interesting interactions with Sonic and his friends.
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.
If it's a crossover with the Legend of Korra, Korra herself would be an excellent addition. She is a powerful bender and has a lot of experience in fighting evil. She and the Trollhunter could learn from each other's fighting styles. Asami could also be included, providing her engineering skills to help the Trollhunter with any technological needs in their battles against the common foes.
For a crossover with a post - apocalyptic world, characters like Mad Max could be involved. He might see the homunculi as either allies or threats in the harsh new world. In a crossover with a cyberpunk world, characters like Neo from 'The Matrix' could be interesting. The homunculi could be part of the complex digital and physical world that Neo navigates, and he could help them understand the true nature of their existence in this new, high - tech world.
Hermione Granger from 'Harry Potter' could be an interesting addition. She could use her magic knowledge to study the mummy. Maybe she could find a spell to control or at least understand the mummy better. It would be a really unique combination of magic and ancient Egyptian mystery.
In a crossover with 'Fullmetal Alchemist', Edward Elric could be an interesting addition. His scientific knowledge and his search for the Philosopher's Stone could interact with the strange alchemy - like aspects of Rea's resurrection in Sankarea. He might be both fascinated and appalled by what has happened to Rea.