Dr. Eggman could team up with a Pokemon villain like Team Rocket. They could combine their technology and evil plans. Eggman might use Team Rocket's knowledge of Pokemon to capture strong Pokemon and use them in his machines, while Team Rocket could benefit from Eggman's advanced robotics to create more powerful devices for stealing Pokemon.
In a 'sonic meets pokemon fanfic', it could be interesting if the Pokemon themselves gain some of Sonic's abilities. For example, a group of Pikachu learn to run at Sonic - like speeds after being exposed to some chaos energy. This new ability changes the way Pokemon battles work in the region, and Sonic has to help the Pokemon trainers adapt to these changes while also dealing with an antagonist who wants to use this new phenomenon for their own evil purposes.
Dr. Eggman could be a great main villain. He might have a new plan that involves both the power of Sonic Underground's music and the Chaos Emeralds that Shadow guards. He could try to create a device that amplifies negative emotions through music and uses the Chaos Emeralds' power to spread chaos everywhere.
It could be Eggman. He might team up with some of the Justice League's villains, like Lex Luthor. Eggman could bring his mechanical creations and knowledge of chaos emeralds, while Luthor provides his strategic mind and resources. Together, they could pose a huge threat to both Sonic and the Justice League.
It could be a new villain created just for this fanfiction. Maybe a powerful wizard who wants to use the combined powers of Sonic Underground and Tails for his own dark magic. He sees their unique abilities as a way to gain ultimate power and they have to fight against his spells and minions.
Maybe there's a new character, a mad scientist who has been observing both the Sonic and Bendy universes. He creates a device that can merge the two worlds in a chaotic way, causing disasters in both. He believes that by doing this, he can become the ruler of the combined universe. Bendy and Sonic must stop him before the two worlds are completely destroyed.
In a 'sonic meets pokemon fanfiction', Sonic could end up in the Pokemon world and start racing against some of the faster Pokemon like Rapidash. Maybe he'd form an unlikely friendship with Pikachu, with Pikachu using its electric attacks to help Sonic out of tough situations, like zapping robotic enemies that Sonic might encounter in this new world.
Team Rocket would be a likely main villain. They are a big part of the Pokemon Red and Blue story, and when Ash enters this world, they would surely cause trouble for him. Their schemes to steal Pokemon and take over the world would be a direct threat to Ash and his new friends.
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.
Maybe the main villain is a mutant who has turned evil and has a grudge against both the wizarding world and normal mutants. He wants to destroy the balance between the two worlds. He sees Harry as a symbol of the wizarding world's power and Logan as a threat to his plans of mutant domination. So he sets out to eliminate them both.
Another possibility for main characters could be Rainbow Dash from MLP and Blastoise from Pokemon. Rainbow Dash is brave and loves adventures, which would be perfect for exploring the new Pokemon world. Blastoise is strong and reliable. They could form a great team, with Rainbow Dash using her speed to scout ahead and Blastoise using its water attacks to protect them from danger.