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.
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.
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.
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 addition to the main characters, characters like Knuckles and Toad could play important roles. Knuckles could be the muscle, protecting the group from physical threats. Toad could be the guide, showing Sonic around the Mushroom Kingdom. And of course, Dr. Eggman and Bowser should be there as the main antagonists. Their combined evil plans would be what Sonic and Mario have to overcome.
In a Dorohedoro crossover with 'One Punch Man', Saitama would be an interesting addition. His overpowered nature would contrast with the often - more - gritty and complex power levels in Dorohedoro. Another good character could be Guts from 'Berserk'. His dark and brooding personality would fit well in Dorohedoro's gloomy atmosphere. Also, Edward Elric from 'Fullmetal Alchemist' could bring his knowledge of alchemy and his quest for truth to the Dorohedoro world.
In vegito crossover fanfiction, characters like Jiren would be great. His strength and stoic nature would make for some intense interactions with Vegito. Also, Android 18 could be included. She has her own unique set of powers and a strong will. Her presence could add some interesting dynamics, especially if she has to team up with Vegito against a common enemy. And don't forget Frieza. His evil nature and thirst for power always make for an exciting addition to any Dragon Ball - related story.
Kenny McCormick (Mysterion) could cross paths with Batman. Batman's detective skills and high - tech gadgets would make for an interesting dynamic with Mysterion's more 'underground' superhero style. They could work together to solve a mystery that spans Gotham City and South Park.
For a crossover with Doctor Who, the Doctor could be an interesting addition. He could bring his time - traveling abilities and vast knowledge to the Shadow World. And from the Shadowhunter side, Jace Wayland would be a good character to include. His charm and combat skills would make for some interesting dynamics with characters from other universes.