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 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 possible plot is that Sonic is exploring a new area. He comes across a small workshop where Tails is building his first plane. Sonic is intrigued by the little fox and his inventions. Tails is initially startled by Sonic's sudden appearance but then gets excited to meet his idol. They start chatting about their adventures, with Sonic sharing his tales of running fast and Tails talking about his love for inventing things.
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.
They might go on an adventure. Maybe they'll discover a hidden treasure or face a new enemy together.
In a fanfiction where Tails meets Sonic for the first time, Tails might be in awe. Sonic's speed and heroic reputation could make Tails instantly idolize him. Maybe Sonic saves Tails from a dangerous situation, like a group of badniks. Tails, being the smart fox he is, could then offer to help Sonic with his gadgets, starting a great friendship.
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.
One possible plot could be that Sonic yells at Tails because Tails made a risky decision during a mission that almost put them both in danger. For example, Tails might have tried to take on a powerful enemy alone without waiting for Sonic's backup.
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.
A possible plot could be that Sonic from Sonic X and Sonic from Sonic Underground meet and initially have a bit of a rivalry over who is the faster or better Sonic. Then they realize they need to work together to stop an evil force that is threatening both their worlds. Another idea is that the characters from Sonic Underground are lost in the world of Sonic X and they have to find their way back home while dealing with the new threats and friends they encounter there.