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.
A very different ending could be that Tony and Loki form a pact. Tony promises to help Loki get something he wants (like a chance at redemption or a certain magical item) in exchange for his freedom. And this pact changes the dynamics of the whole Avengers - Loki relationship, leading to new storylines and adventures in the future.
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.
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.
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.
Thor would be furious. He would immediately start looking for Jane, using all his resources. He might first try to reason with Loki, but if that doesn't work, he would not hesitate to use force.
Maybe he uses his magic. Loki is a powerful sorcerer. He could have created a portal or used some sort of teleportation spell to get to Siberia quickly.
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.
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.
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.
It could mean that Tony, in the context of fan - created stories about Loki, acts in a way to safeguard or be defensive about them. Maybe he doesn't want others to criticize or misinterpret the Loki fanfiction.
Maybe Tony notices some unusual behavior in Loki. For example, Loki might be acting against his normal self - not as cunning or devious as he usually is. His actions could seem more robotic or forced, which makes Tony suspect that he's being controlled.