One idea could be that they initially struggle to adapt to each other's bodies. Tails in Amy's body might find it hard to handle her more emotional nature and her way of interacting with Sonic. He could accidentally make things awkward when trying to be Amy. For Amy in Tails' body, she might be excited to use his inventions but quickly realizes she doesn't have the same technical know - how. They could then have to work together to figure out how to reverse the bodyswap while also learning more about each other in the process.
Sonic would probably be very confused at first. He might mistake Tails in Amy's body for the real Amy and be really surprised by her different behavior. Knuckles would likely be suspicious and keep asking questions. Cream might be worried about her friends and try to help them figure out what happened. Big the Cat probably wouldn't notice much difference at all, just going about his normal fishing routine.
One interesting Amy and Tails fanfiction could be about their adventure in a new, uncharted zone. Maybe they discover a lost civilization and have to work together to solve the mysteries there. In this story, Amy's bravery and Tails' intelligence combine to overcome various challenges like deciphering ancient runes and avoiding dangerous traps.
Maybe in the fanfiction, the three - tailed beast crosses over into a modern - day human world. People initially fear it, but then they discover that it has a gentle side and is actually trying to find its way back home. It could form a bond with a group of humans who help it on its journey, leading to many adventures along the way.
One idea could be exploring the origin of the Ten - Tails in a different light. Maybe it was created by an ancient civilization for a noble purpose that got corrupted over time. Another could be a story where a young ninja discovers a hidden power within the Ten - Tails and has to learn to control it without being consumed by its dark energy.
One idea could be that Fluttershy discovers Tails' mechanical skills and they work together to build a special sanctuary for injured animals in both their worlds. This would combine Fluttershy's love for animals and Tails' engineering prowess.
Another plot could be Tails getting lost in a dimension where all the laws of physics are different. His gadgets don't work as expected, and he has to rely on his wits and basic survival skills. He could meet some new friends or foes in this dimension and try to find his way back home while also learning about this strange new place.
One idea could be that Luigi and Tails team up to solve a mystery in a strange, hybrid world that combines elements from their respective universes. They could use Tails' mechanical skills and Luigi's courage to face unknown challenges.
One plot idea could be Naruto struggling to control the 4 - tails form in a new and dangerous mission. Maybe he has to fight an enemy that specifically targets his unstable state. Another could be his friends trying different ways to help him calm down when he goes 4 - tails, leading to some heart - warming moments of friendship.
One idea could be that Sonic and Tails are camping out in a peaceful meadow. They fall asleep under the stars and have shared dreams of their future adventures together. In the dream, they discover a new power - like the ability to fly even faster. Then they wake up excited to start the day and see if they can make that dream a reality.
One plot idea could be that an earthquake hits Sonic's world and Tails uses his inventions to help the inhabitants. For example, he could create special devices to stabilize buildings or detect aftershocks early. Another idea is that the earthquake is caused by an evil villain's new weapon, and Sonic and Tails have to stop the villain and also deal with the destruction. Or perhaps Tails discovers that there are ancient underground structures that the earthquake has unearthed, leading to a new adventure.