Who could be the main villain in a Sonic Underground meets Tails fanfiction?

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.

In this fanfiction, perhaps Sonic Underground is facing a threat that they can't handle alone. Tails, being a friend of Sonic from another part of their world, comes to their aid. They discover that an ancient evil has been awakened. The story could involve Tails using his inventions to analyze the ancient evil's weaknesses while Sonic and his siblings use their unique abilities to keep the evil at bay. As they progress, they form a strong bond and learn from each other's ways of doing things.

Sonic is often paired with Tails. Their friendship in other Sonic canons carries over, and in Sonic Underground fanfiction, they might have new adventures together. Sonic's speed and Tails' smarts make them a great team.

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.

One possible plot could be that the characters from Sonic Underground get transported to the world of Sonic X. They then team up with Sonic and his friends in Sonic X to fight a common enemy like Dr. Eggman who has somehow become even more powerful by combining technologies from both universes. The different Sonic characters would have to learn to work together, sharing their unique abilities and skills.

One common theme is friendship. Tails often forms strong bonds with the characters from Sonic Underground, and these relationships are explored in many fanfictions. For example, how he becomes a trusted ally and friend to Sonic, Sonia, and Manic.

Sonic himself is usually really popular. Whether it's the Sonic from Sonic X or Sonic Underground, he's the main hero and often at the center of the action. People love reading about his adventures and how he deals with the challenges in the combined universes.

One interesting story could be about Tails using his mechanical skills to upgrade the Sonic Underground's vehicles. He might create some high - tech gadgets that help them in their adventures against Dr. Robotnik.

In the fanfiction, when Sonic meets Tails in the snow, they could also just have a friendly chat. They might talk about their past adventures, like the time they saved the world from Dr. Eggman. Tails could share some new inventions he's been working on, and Sonic would be excited to hear about them. They might also plan their next adventure while enjoying the snowy scenery.

Dr. Eggman would be a likely main villain. He might be the one who devises a plan to make Sonic fat, thinking that it will slow him down and make it easier for him to be defeated.