Link from The Legend of Zelda would also be a cool crossover. Link's swordsmanship and Sonic's speed could combine in interesting ways. Maybe Sonic helps Link travel faster between dungeons, and Link protects Sonic from more close - range threats. They could work together to defeat Ganon who has somehow teamed up with Dr. Eggman.
Tails would be an obvious choice as he is already part of Sonic's world, but if we think outside the box, characters like Spider - Man could be great. Spider - Man's acrobatic skills and Sonic's speed would make for an exciting team - up. They could swing and run through New York City together fighting crime.
Mario could be a really interesting crossover. The contrast between Mario's more platform - based adventures and Sonic's high - speed runs would create unique scenarios. They could compete in races or team up to save both the Mushroom Kingdom and Sonic's world from a common enemy like Bowser and Dr. Eggman working together.
Alucard from "Hellsing" would be an interesting character to cross over with Seto Kaiba. Their both powerful and have a certain arrogance. They could have an epic battle or end up teaming up against a common enemy. Another good one would be Light Yagami from "Death Note". The clash of their intellects would be fascinating, as Kaiba tries to outwit Yagami's plans. Also, Spike Spiegel from "Cowboy Bebop" could be a great crossover. Their different personalities would create a lot of interesting interactions.
Sonic would definitely be one of the main characters. His speed and heroic nature would make him a key figure in any crossover. Also, Freddy Fazbear from FNAF would be important as he is sort of the face of the FNAF franchise. Sonic could be the one trying to find a way out of the FNAF world or stop the animatronics, and Freddy could be the character that Sonic has to either defeat or befriend in order to achieve his goal.
Eggman Nega could be a great main antagonist. He has the intelligence and the desire for power. He could find a way to open portals between the Sonic X and Sonic Boom universes to steal resources and technology from both. His plans would force Sonic X's Sonic and Sonic Boom's Sonic to team up against him. He could use his robotic creations from both universes, combining the advanced tech from Sonic X and the more rustic, yet effective, machines from Sonic Boom.
Miles Morales could have an amazing crossover with Daredevil. Daredevil's enhanced senses and martial arts skills combined with Miles' spider - powers would be a great combination. Also, Gwen Stacy as Spider - Woman could cross over with Black Widow. Their skills in combat and acrobatics would make for an exciting team - up. And Peter B. Parker could cross over with Captain America. Cap's leadership and Peter's experience as Spider - Man would create a really interesting dynamic.
I think characters from 'Fairy Tail' would make for great crossovers. The Fairy Tail guild members are all about friendship and adventure, which could mesh well with the story of 'Welcome to demon school'. Also, characters from 'Naruto' would be interesting. The demon students could learn about the concept of ninja values like perseverance and loyalty. They could also have some epic battles with the ninja.
Sonic could teach some of the Pokemon his speedy moves. For instance, he could show Pikachu how to move even faster during battles. In return, the Pokemon could share their unique abilities with Sonic. A water - type Pokemon could teach Sonic how to swim better in case he encounters a large body of water in his adventures. They could also form teams to take on common enemies. Sonic and his friends, along with their Pokemon partners, could fight against Dr. Eggman and Team Rocket if they were to team up in the fanfic.
A prevalent theme in sonic crossovers fanfiction is the fight against a common enemy. Whether it's an evil force from Sonic's world or a new threat created by the crossover, characters from different franchises join forces. Also, the theme of discovery is common. Sonic and the other characters discover new places, cultures, and ways of life in the crossover stories.
One great example could be a story where Sonic teams up with Archie characters to fight a common enemy like Dr. Eggman and some Archie - specific villain. In such a fanfiction, the writers might explore how Sonic's speed and the unique abilities of Archie characters combine to overcome challenges.
One idea could be a Sonic crossover with a superhero universe. Sonic's speed could be compared to the Flash's, and they could team up to stop a common villain who is trying to steal chaos emeralds and speed - force related items. Another plot could be a crossover with a fantasy world. Sonic meets magical creatures and has to use his speed to outrun dark magic. Maybe he teams up with a wizard or a fairy.
Hulk might be an interesting one too. Sonic's speed could either annoy or amuse Hulk. If they end up on the same side, Sonic could use his speed to lead Hulk towards the enemies. And Hulk's raw strength could be something Sonic would be in awe of. They could have a really unique dynamic, like a fast - moving pest annoying a big, strong giant at first, but then they become an effective team.