A Sonic Mania crossover with Kirby could also be great. Imagine Sonic and Kirby meeting in a dream - like world. Kirby could copy Sonic's speed ability, while Sonic could use Kirby's power of flight (temporarily). They'd have to face off against a combined threat of Eggman and some of Kirby's villains like King Dedede, who's been tricked by Eggman into thinking Sonic is a threat to his kingdom.
Another plot could involve a crossover with the Pokémon world. Sonic and Tails could find themselves in a Pokémon region. They'd have to work with some Pokémon trainers to stop Eggman from using Pokémon for his evil plans. Sonic's speed would be useful for quickly getting around the region, and his spin attack could be used in battles, while Tails could use his inventions to help the trainers understand new Pokémon abilities.
One idea could be a crossover with another classic game series like Mega Man. Sonic could team up with Mega Man to stop Dr. Eggman and Dr. Wily who have joined forces. They'd combine their unique abilities, Sonic's speed and Mega Man's weaponry, to take on the evil duo in a world that combines the zones from Sonic Mania and the levels from Mega Man games.
One idea could be Sonic crossing over with another popular game like Mario. Sonic could find himself in the Mushroom Kingdom and have to team up with Mario to stop a new threat that combines the chaos emeralds and power - ups from both worlds. Maybe Bowser has teamed up with Dr. Eggman to create this chaos.
One idea could be a crossover with another popular game character like Link from The Legend of Zelda. Sonic the Werehog could find himself in Hyrule and have to team up with Link to defeat a common enemy. Maybe Ganon has somehow tapped into the power of the Chaos Emeralds and Sonic's werehog form is needed to counter it due to its unique strength and agility in that form.
Well, a cool plot for Mario Sonic crossover fanfiction might involve a new character that can travel between their worlds freely and is causing chaos. Mario and Sonic initially suspect each other but then realize they need to join forces. For example, this character could be stealing power - ups from Mario's world and Chaos Emeralds from Sonic's world to gain ultimate power. They would have to explore both worlds, facing their respective villains' minions along the way.
One idea could be that Mario and Sonic have to team up to save both their worlds from a new, powerful villain that is threatening to merge their dimensions in a chaotic way. For example, Bowser and Dr. Eggman could accidentally awaken an ancient evil while arguing over who is the better baddie.
One idea could be Sonic and his friends from the SatAM universe teaming up with characters from another Sonic series to defeat a new, powerful villain that threatens both their worlds. For example, they could join forces with the Sonic Boom crew. This would bring together different versions of Sonic characters with unique abilities and personalities, leading to some really fun interactions and epic battles.
One idea could be that they have to team up to stop a new powerful villain who is causing chaos across different zones. This villain could be creating some sort of energy disruptions that only their combined powers can counter.
Shadow the Hedgehog would be an interesting addition to a crossover. In a crossover with the DC Universe, he could be compared to Batman. Shadow's brooding nature and combat skills could make him a formidable ally against villains like the Joker and Dr. Eggman. He could use his Chaos Control ability to quickly move around Gotham City or Sonic's zones, just like how Batman uses his gadgets to traverse the city.
One idea could be that Sonic gets transported to the human world and has to adapt to a normal high school life while still trying to keep his super - speed a secret. He could make friends with some humans who gradually discover his true identity and help him fight against an evil force that has also made its way into the human world.
One idea could be that Sonic and his friends accidentally stumble into the world of the Transformers. Sonic's speed could be a valuable asset in a battle against the Decepticons. For example, he could quickly deliver important messages or parts between Autobots during a fight. Another idea is that the Chaos Emeralds somehow have an impact on the AllSpark, creating new powers or challenges for both the Sonic characters and the Transformers.
One idea could be that Sonic accidentally enters the world of Equestria Girls while chasing Dr. Eggman through a strange portal. He then teams up with the girls to stop Eggman from stealing the magic that powers their world. Another could be a race between Sonic and Rainbow Dash which gets interrupted by a common enemy.