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.
One popular plot is Sonic and Tails going on an adventure to save their world from a new, powerful villain. They face many challenges together, relying on each other's unique skills. For example, Sonic's speed and Tails' intelligence are combined to solve puzzles and defeat enemies.
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.
Their interaction would likely be positive. Sonic, being the friendly hero, would probably greet Tails with a big smile. Tails would be shy at first but soon open up due to Sonic's warm attitude.
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.
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.
There might be a fanfiction where Tails and Sonic time - travel. They could end up in a different era in the Sonic universe and have to find their way back while also dealing with the problems of that time period. For example, they might have to face an ancient evil that was thought to be long gone.
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.
First, come up with a unique idea. It could be something like a day in the life of Sonic and Tails that's different from the usual adventures. Then, create a simple outline of the beginning, middle, and end. For example, start with Sonic and Tails waking up to an unusual event.