A more complex plot could be that de - aged Tony and Loki have their memories intact. They use their knowledge of the future (in Tony's case) and Loki's magic to change some events that would have had a negative impact on the world. But they have to do it in secret because no one would believe two 'kids' could be so knowledgeable.
Another plot might involve the Avengers trying to figure out how to deal with de - aged Tony and Loki. They could be worried about Loki's intentions since he's usually a villain, but at the same time, they have to take care of a child - like Tony. This creates a lot of comical and tense situations.
Fanfiction.net is also a place to look. There are many writers who contribute to different fandoms. However, the search function might not be as advanced as AO3. But still, it's worth a try to find 'de aged tony and loki fanfiction'.
One interesting plot could be Steve adjusting to the new, younger body while still having the mind of an older hero. He might struggle with modern technology even more than before, leading to some comical situations. For example, trying to figure out a smartphone or a new type of vehicle.
One interesting plot could be Peter Parker de - aged to a child and having to navigate his old life with his current knowledge. For example, he might struggle to hide his superhero identity from his younger self's friends and family. Another plot could involve him going back to school as a child while still having his adult - level intelligence, which could lead to some comical and also poignant situations as he tries to fit in with kids his age.
One possible plot could be Loki initially trying to use Tony for his own gain but then truly falling for him. Maybe Loki, in his usual sly way, gets close to Tony to access some of Stark's technology, but as time passes, he discovers Tony's charm, intelligence and vulnerability, and his feelings turn sincere.
A possible plotline is that the de - aged Peter and Tony have to work together to solve a problem that only their combined knowledge, both from the present and Peter's future - self's knowledge, can fix. For example, they might need to stop a villain who has a time - related weapon that de - aged Peter in the first place. This would involve Peter using his spidey - sense in a different way as a kid and Tony using his tech skills to analyze the weapon and figure out how to reverse its effects not just on Peter but also on any other potential victims.
There are several popular ones like 'The Youthful Adventures of Thor and Loki' which has a great plot and character development.
One common theme is the exploration of Tony's lost innocence. When he's de - aged, he gets a chance to experience things in a new light. Another theme is the impact on his relationships. For instance, his relationship with Pepper might take on a different dynamic as he's now younger. Also, there's often the theme of self - discovery. Tony may find out new things about himself that were buried under years of being an adult.
One possible plot could be that Tony, due to some external pressure or misunderstanding, gets involved with another character while in a relationship with Loki. Maybe he's forced into a political alliance that looks like cheating to Loki. Another could be that Tony's self - destructive tendencies lead him to make a mistake and cheat, causing a rift in their relationship that they then have to struggle to repair.
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.
The de - aged Tony could also form a mentor - mentee relationship with a younger Avenger. He has the knowledge and experience of his older self but the energy and fresh perspective of his de - aged self. This relationship could be a key factor in how the story progresses, as he imparts his wisdom in a different way and the younger Avenger helps him see things from a new angle, which could potentially change the course of the Civil War in the fanfiction.