Secondly, create a unique setting. Instead of the usual battle - torn Cybertron, set the story in a parallel universe where the Autobots and Decepticons have different roles or in a post - apocalyptic Cybertron where they have to rebuild together. This can add a fresh perspective to their relationship.
One possible storyline could be a redemption arc for Megatron. In this fanfiction, Megatron, after countless battles with Optimus Prime, begins to question his motives. He realizes the destruction he has caused and tries to make amends. Optimus, initially skeptical, gradually sees the change in Megatron and they form an uneasy alliance to face a greater threat that endangers both Autobots and Decepticons alike.
Perhaps there's a story where Optimus and Megatron are both stranded on a desolate planet. Without their armies and with limited resources, they have to rely on each other to survive. As they struggle to find a way off the planet, they start to understand each other better. Megatron realizes that Optimus isn't just a naive idealist and Optimus sees that Megatron has reasons for his actions that are more complex than just pure evil.
It could be developed by exploring their past. Maybe there was an event in their youth that they both remember differently. As they start to talk about it in the fanfic, they begin to see each other in a new light. For instance, they might have once been friends or at least comrades in a fight against a common enemy before their ideologies split them apart. This exploration of the past can build a more complex relationship between them.
Fanfiction about them can explore different universes and scenarios. For example, what if they were raised together? How would their relationship be different? It gives fans a chance to play with the established lore and create new and exciting stories that the official Transformers storylines may not explore. It also allows for deeper exploration of their motives and personalities.
You can try looking on some fanfiction websites like Archive of Our Own (AO3). It has a large collection of Transformers fanfictions, and there might be some related to Optimus Prime and Megatron in the MPreg genre.
In a fanfic about Optimus Prime x Megatron, their relationship could be explored in a more philosophical way. For example, they might debate the nature of good and evil, power and responsibility. Optimus believes in protecting all life, while Megatron has his own ideas about power and how it should be used. This kind of fanfic could show how their different viewpoints could either bring them closer together or drive them further apart.
A great angle for a Megatron x Optimus Prime fanfic could be a body - swap situation. They are forced to inhabit each other's bodies due to some alien technology gone wrong. This leads to them having to understand each other's perspectives on a whole new level. They start to see the reasons behind their actions in the past and develop a new - found respect for one another.
One possible fanfiction could be about Optimus Prime and Megatron having to team up against a greater threat. Maybe an ancient evil from Cybertron's past resurfaces that is so powerful that their usual enmity has to be put aside. They start off distrusting each other but as they face battles together, they gradually come to respect the other's fighting skills.
Baby Optimus Prime's character can be developed by exploring his relationships. If he has a mentor figure among the Autobots, his interactions with that character can shape him. He might look up to this mentor and try to imitate their good qualities. His relationship with other baby Transformers or younglings can also be important. He could learn about sharing, cooperation, and friendship through these relationships. Additionally, his discovery of his own strength, both physical and mental, can be a key part of his development. For instance, when he first realizes he can lift a small object that others thought was too heavy for him, it gives him confidence.