One possible reason could be the physical injuries he sustained during the fight in Siberia. The intense battle might have left him with wounds that continue to pain him. Another reason could be the emotional betrayal he felt, especially if in the fanfiction his friends turned against him during that event, which would deeply wound him psychologically.
It could be a local Siberian warlord who sees Tony as a threat to his power. Maybe Tony's presence in Siberia disrupts the warlord's plans for the region, so he decides to eliminate him.
One possible scenario could be that Tony is in Siberia on a mission gone wrong. Maybe he's facing a powerful enemy there, and despite his best efforts, he gets fatally injured in a fight. Another could be that the harsh Siberian environment plays a role, like he gets caught in a blizzard and succumbs to the extreme cold.
Other fanfictions might focus on his emotional hurt. He could be depicted as withdrawn, not talking to anyone, and spending long hours alone in his workshop. His trust in others might be completely shattered, and he may be constantly haunted by memories of Siberia, which is shown through his brooding and melancholy behavior.
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.
Well, perhaps it could involve Tony dealing with the physical and emotional challenges of being pregnant in the harsh environment of Siberia. Maybe he has to find ways to get proper medical care while being in such a remote place.
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.
Perhaps in this fanfiction, Tony is kidnapped from Siberia by a powerful villain who has a long - standing grudge against him. The kidnapper could be after Tony's Iron Man suit technology. They snatch him from Siberia as it was a place where he thought he could be safe or was conducting some top - secret research related to his suit's power source. His friends then have to embark on a dangerous rescue mission.
I haven't actually read this specific fanfiction, so I can't summarize it directly. But generally, it might involve Peter using his powers, like his spider - sense and agility, to rescue Tony who could be in some sort of danger in Siberia. Maybe there are battles against villains or difficult situations that Peter has to overcome to save Tony.