One idea could be Sonic and his friends accidentally end up in the Pokemon world. Sonic's speed could be a unique ability in Pokemon battles, like a super - fast attack or a way to quickly dodge enemy moves. For example, he could use his speed to outrun a powerful Pokemon's attack and then counter - attack. His friends like Tails could use his inventions to interact with the Pokemon technology, perhaps creating new Pokeballs or devices.
A common theme is the clash of two different worlds. Sonic comes from a world full of natural landscapes and speed, while Megaman is in a world of technology and robots.
One interesting concept could be the combination of different Sonic universes. For example, merging the classic Sonic games' world with the more modern Sonic Boom universe. This can lead to unique character interactions, like Sonic from the classic era meeting Sticks from Sonic Boom.
Character interactions are key. For example, how Sonic would interact with characters from another universe like how he'd get along with a character like Pikachu in a Pokemon crossover. It should also have a coherent plot. If it's a Sonic x Star Wars crossover, there should be a clear reason for Sonic being in the Star Wars universe and what their goals are.
Sonic would probably be really excited to meet Rainbow Dash. He might challenge her to a friendly race right away. Rainbow Dash, being competitive herself, would accept without hesitation.
One idea could be a racing tournament where Mario, Sonic and their friends compete. It would be a high - speed adventure with crazy power - ups from both universes. Maybe Mario could use a Sonic - inspired speed boost item, and Sonic could try out a mushroom to grow big for a short time.
First, decide which Sonic elements you want to use. Do you prefer the old - school Sonic or the new versions? Then, choose the other franchise or element you want to cross it with. After that, think about a basic plot. Maybe start with a problem that only the combined powers of the Sonic and the other element can solve.
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.
Another great sonic x crossover fanfic is 'Sonic x Harry Potter: Magical Races'. In this, Sonic and his crew find themselves in the wizarding world of Harry Potter, leading to some really fun and unexpected situations as they interact with wizards and magical creatures.
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.