One possible plot twist could be that Loki actually has a noble reason for kidnapping Tony. Maybe he wants to use Tony's intelligence to stop a greater threat that the other Avengers are unaware of. For example, there could be a hidden enemy that Loki has knowledge about, and he believes only Tony can help create a device or strategy to defeat it.
It could also end with a more complex resolution. Loki realizes the error of his ways during the time he has Tony kidnapped. Maybe through their conversations, Loki sees the value in Tony's life and the good he does. So, Loki releases Tony and decides to help the Avengers in their fight against a common enemy as a form of redemption.
One possible scenario in this fanfiction could be that Loki disguises himself and lures Spiderman into a trap. He then transports him to a different realm where Spiderman is completely out of his element. Spiderman would have to rely on his wits and basic powers to try and escape Loki's clutches.
One possible outcome in this fanfiction could be that Loki kidnaps Darcy for a very specific reason. Maybe he needs her knowledge of some scientific concept that is relevant in the world he is from or is planning to use in his schemes. For example, if Darcy has been working on some sort of energy research that Loki believes can be harnessed for his own power - enhancing purposes.
If Loki kidnaps Spiderman in fanfiction, it would be a very interesting plot. Loki is a powerful and cunning character, while Spiderman is a beloved hero. Loki might underestimate Spiderman's resourcefulness though. Spiderman could use his spider - sense to find weaknesses in Loki's plans.
In the fanfiction, Loki may have developed a sense of respect or even friendship for Tony. They are both very intelligent characters. Loki might admire Tony's ingenuity and courage. So when he realizes that Tony is in danger in Siberia, his new - found feelings for Tony drive him to save his life. Also, Loki might be bored with his usual villainous ways and wants to do something good for a change, and saving Tony seems like a good start.
It could be someone who is manipulating Loki to kidnap the Hulk. Maybe an even more powerful villain from the Marvel universe, like Thanos. He might be using Loki as a pawn to get the Hulk for his own evil purposes.
Tony would likely be defiant. He's not the type to cower in the face of danger. He might immediately start looking for weaknesses in Thanos' setup and try to escape on his own terms.
At first, Bruce might be scared. He knows Loki's reputation for being unpredictable and dangerous. But he also tries to keep his cool because he doesn't want to accidentally transform into the Hulk right away. He might try to reason with Loki, using his intelligence to talk his way out of the situation.
At first, it would be rocky. Loki is used to being a trickster and Tony is a bit of a control freak. But over time, they'd start to respect each other. Loki would see Tony's intelligence and resourcefulness, and Tony would see Loki's power and potential for good. Eventually, they'd become like family, looking out for each other.