One popular trope is 'enemies to lovers'. Loki and Tony start off as adversaries, often at odds due to their different personalities and loyalties. But as the story progresses, they gradually fall in love. Another is 'soulmates'. In some fanfictions, they are fated to be together in some magical or cosmic way. 'Hurt/comfort' is also common, where one of them is injured or in distress and the other provides care and support.
One popular trope is Loki as a 'fish out of water' in the Star Wars universe. He's used to the world of Asgard and suddenly finds himself in a completely different setting with different rules and species. It leads to a lot of comical and interesting situations.
One popular theme is the redemption of Loki. In many fanfictions, Thor and Tony play important roles in Loki's journey to redeem himself. For example, they might help him face his past mistakes and find a new purpose.
One popular theme is the exploration of their relationship dynamics during the pregnancy. It could be about how they support each other through the unexpected situation. Another theme might be the reactions of the other Avengers to this rather extraordinary event. Some fanfictions also focus on the challenges they face in terms of Loki's Asgardian nature and Tony's human - tech - influenced body during the pregnancy.
One popular theme is redemption. For example, Loki might be seeking redemption for his past misdeeds, and Tony and Bucky, who also have their own issues in the past, can relate to his journey in some ways. They help each other grow and move forward.
One popular trope is the 'hurt/comfort' where either Loki or Mobius gets injured or is in emotional distress, and the other comes to their aid. For example, Loki might get hurt during a battle in a different timeline, and Mobius would be there to comfort and heal him. It shows their closeness and care for each other.
One popular trope is the 'enemies - to - friends - to - lovers' arc. Given their complex personalities, they could start as rivals or even enemies, perhaps due to their different natures (Lucifer being a fallen angel and Loki being a Norse god of mischief), and gradually develop a deep connection. Another is the 'fish - out - of - water' trope, especially if they are placed in each other's realms. For example, Loki in Hell or Lucifer in Asgard.
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.
One popular trope is the 'fish out of water' where Tony Stark is suddenly in the magical world of Harry Potter and has to adapt. Another is the 'team - up' trope, where they join forces to fight a common enemy, like a dark wizard using technology in a bad way.
One popular trope is the 'Genius Rivalry'. Your OC can be another brilliant mind who either clashes or collaborates with Tony Stark. It adds a lot of tension and interesting exchanges. Another is the 'Underdog' trope. Maybe your OC is a new recruit to the Avengers with no real super - powers but a lot of heart and determination. They have to prove themselves to Tony and the others. 'Love Interest' is also common. Your OC could fall in love with Tony, or vice versa, which can lead to some sweet and sometimes angsty moments.
The 'family - like bond' trope is often seen. De - aged Tony forms closer bonds with the people around him, almost like a family. They protect him and he in turn, uses his genius to help them. For instance, he might become like a little brother figure to some characters. And there's also the 'coming - of - age' trope even though he's already an adult. He still has to grow in this new de - aged state, learning new things about himself and the world.