A common theme is adventure. Both Sonic X and Sonic Underground are about Sonic and his friends going on exciting adventures, and this is often carried over into the fanfiction. For example, they might be on a quest to find a powerful chaos emerald or save a new world from an evil villain.
Sure. 'Sonic's New Adventure' is a great one. It combines the elements from Sonic X and Sonic Underground really well. The story takes Sonic on a journey across different dimensions, facing new enemies and making unexpected allies.
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.
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.
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.
One popular sonic underground tails fanfiction is 'Tails' Great Adventure in Sonic Underground'. It has an exciting plot where Tails teams up with the Sonic Underground crew for a new adventure. He uses his intelligence and flying skills to help them overcome various obstacles.
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.
One idea could be that Sonic Underground's siblings get separated in a new zone and Shadow accidentally stumbles upon them. He initially thinks they are enemies but then realizes they are on the same side against a common foe. Another could be that Shadow is on a mission related to the Chaos Emeralds and crosses paths with Sonic Underground. They end up having to team up to stop a new villain who wants to use the power of music and the Chaos Emeralds for evil.
Sonic himself is always popular. His speed and heroic nature are central to many fanfics. Then there's Sally. She is often a key character, especially in stories that involve strategy and leadership. Bunnie is also well - liked. Her mechanical enhancements can add an interesting element to the story. And of course, Antoine, with his comical French accent and sometimes clumsy but brave nature, makes for great character interactions in the fanfic world.
One interesting concept could be new character relationships. For example, exploring how Sonic and his underground friends interact in different scenarios that weren't in the original. Another concept might be alternate storylines. Maybe Sonic has to face a new villain that is specifically related to the underground world. Also, the fanfic could focus on the backstories of the underground characters, like their origin and how they came to be part of the Sonic universe.