Well, it depends on the nature of the fanfiction. In some cases, the kidnapped character's friends or allies will come to rescue them. This can lead to a big battle or confrontation with sans. If sans is being controlled by someone else when he kidnaps, the rescuers might have to figure out how to free him from that control as well. Sometimes, the kidnapped character might also find a way to escape on their own, and then they have to decide whether to get revenge or try to understand sans's motives.
The story could also focus on her solo adventures. She might go on a journey to discover more powerful magic or ancient knowledge that could help in the fight against the Dark forces in a way that the Order didn't consider. This could lead to her becoming a more powerful and self - reliant character.
Often, his friends start to search for him. His best friend Ned might use his tech - savvy skills to try and track him down. They might follow some leads that Peter left behind, like a special code or a hidden message.
Often, his friends will try to find him. Misty and Brock, or his newer friends like Serena, might set out on a journey to track him down. Their search could lead them to new places and new Pokémon they wouldn't have otherwise encountered.
The story could develop in many ways. If it's a more angsty fanfiction, Annabeth might feel betrayed by Percy and their relationship would be severely strained. They would have a lot of emotional turmoil to work through. Meanwhile, the outside world would be in chaos. The gods might get involved, seeing as two of their powerful demigod children are in a strange situation. Percy would have to find a way to make things right, either by undoing what he did or by justifying his actions in some way.
The story could develop with Tony trying to reason with Steve. Tony might use his intelligence to talk Steve out of whatever his plan is. He could point out the flaws in Steve's reasoning or show him evidence that he's been misled.
At first, their relationship would be extremely tense. Annabeth would be angry and confused about Percy's actions. However, as time passes and they start to communicate, they might begin to understand each other's motives better. If Percy had a good reason for the kidnapping, like protecting her from an unseen danger, Annabeth could start to see him in a different light.
It could end with Lily escaping and being forever angry with Snape. She might cut all ties with him and go on to marry James Potter, with Snape left full of regret for his actions.
Often, Peter will go off on his own path. He might start to form his own team or work independently to prove himself. For example, he could start taking on smaller villains in his neighborhood to show that he doesn't need Tony's support.
In other cases, Harry might create his own new world, a sort of hybrid between the wizarding and Muggle worlds. He could gather other outcasts from the wizarding world who also want a different way of life, and together they build a community where they can use magic freely but also incorporate Muggle values and ideas. This new community becomes a haven for those who are tired of the traditional wizarding world's ways.
Often, his friends in Konoha will start to search for him. They might split into teams and scour the surrounding areas. For example, Sasuke could use his Sharingan to look for any signs of Naruto's departure. Meanwhile, back in Konoha, there could be unrest as the villagers start to worry about the safety of the village without Naruto.