Another possibility is that an Asgardian traitor, other than Loki, could be the main antagonist. This character might be jealous of the Hulk's power or wants to use him to gain power in Asgard. They could be secretly working with Loki, but in the end, they turn on Loki and try to take the Hulk for themselves. For example, a lesser - known Asgardian who has always been in the shadow of Thor and Loki but sees an opportunity with the Hulk.
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.
Well, perhaps Loki kidnaps the Hulk to use his strength in some scheme against the Avengers. Maybe he wants to use the Hulk to break into a secure location or as a weapon against Thor.
It could be another powerful deity from either the Norse or the Christian pantheon who doesn't want Gabriel and Loki to team up. Maybe a more rigid and traditional angel who thinks Gabriel is consorting with the wrong crowd by interacting with Loki.
An interdimensional being that wants to use Loki's unique connection to different realms. This being has the power to manipulate time and space, making it extremely difficult for the Avengers to reach Loki. The Avengers would need to figure out the being's weaknesses and use their own combined powers and skills to overcome it and rescue Loki.
It could be Malekith. He might see the sister as a way to get to Thor and Loki and gain power over Asgard. Since she's a new and potentially vulnerable target, he could try to use her in his dark schemes.
Perhaps a group of inter - dimensional bounty hunters. They've heard about Loki's children and the potential power they might possess when they grow up. So they target the children to either sell them to the highest bidder or use their powers for their own evil purposes. Tony then has to fight against this group to save the kids.
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.
Another plot twist could be that the reader has some sort of power or knowledge that Loki needs. So, instead of a malicious kidnapping, it's more of a strategic move on Loki's part. He might be trying to bargain with the reader to get what he wants, but as time passes, he starts to develop feelings for the reader.
One possible plot could be that Loki kidnaps Natasha to use her as a bargaining chip against the Avengers. He might think that her skills and the Avengers' attachment to her would make them more likely to give in to his demands.
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.
Loki may also kidnap Bruce Banner as a way to get at the Avengers. Since Banner is an important part of the team, taking him hostage could be a strategic move to draw out the other Avengers and then manipulate or defeat them. It could be part of a bigger scheme to gain power or control over Earth or the universe.