Sonic is very likely to be involved. Since Sonic and Tails are close friends, Sonic would surely try to rescue Tails from Shadow. He would be the hero of the story, using his speed and wits to face off against Shadow.
Another plotline might involve Shadow being mind - controlled to kidnap Tails. This could lead to a story where Sonic and his friends have to not only rescue Tails but also find a way to break the mind - control on Shadow. In this case, Tails might be able to use his intelligence to try and communicate with Shadow and find a way out from within the captivity.
Well, in a typical scenario, Sonic would probably rush to save Tails. He'd use his super speed to track Eggman's lair. Maybe he'd find some clues along the way, like a piece of Tails' fur or a gadget that Eggman accidentally dropped. Then, Sonic would face off against Eggman's badniks, defeat them one by one, and finally reach Tails. He'd free Tails and they'd escape together, with Eggman vowing revenge as usual.
Sonic would be the first to react. He'd be angry and immediately start the search for Tails. Amy would be worried sick and might try to help Sonic in any way she can, like gathering information from the other animals in the area. Knuckles would be on high alert, thinking this could be part of a bigger plan to get to the chaos emeralds. He'd guard the emeralds even more closely while also keeping an eye out for any signs of Eggman's activities related to Tails' kidnapping.
One common theme is friendship. Since they are from different backgrounds in the Sonic universe, seeing how they form a bond in fanfiction is really interesting. For example, Tails' positive attitude can often break through Shadow's tough exterior.
The main characters are, of course, Tails and Shadow. Tails is known for his intelligence and mechanical skills. He is a fox with two tails which he can use to fly. Shadow, on the other hand, is a powerful and often brooding character with super - speed and strength.
I really like 'Shadow and Tails: A New Alliance'. In this fanfic, an external threat forces Shadow and Tails to team up. Tails' intelligence combined with Shadow's power makes for an exciting read. The author does a great job of showing Tails' inquisitive nature as he tries to figure out Shadow's motives, while Shadow slowly starts to open up to the young fox.
Well, it could be that Shadow has some sort of misunderstanding. Maybe he believes Sonic has some information related to a threat to the world. So he kidnaps Sonic to interrogate him. For example, Shadow might think Sonic knows about a new enemy's weakness that Sonic actually doesn't. And Sonic has to convince Shadow of his ignorance while also trying to escape.
It could be Dr. Eggman. He might have manipulated Shadow into kidnapping Sonic. Maybe he promised Shadow something in return, like power - ups for his abilities. Dr. Eggman wants to study Sonic's speed and abilities, so he uses Shadow as a tool to get Sonic in his clutches.
One common theme could be the exploration of an unlikely friendship or partnership between Tails and Shadow. Another might be the growth of their characters as they interact more closely.