Redemption is also a theme. Sometimes Erik has done things that he regrets, and Charles is there to help him find a way back. Or vice versa, where Charles makes mistakes and Erik is the one to offer support. This theme of forgiveness and growth is quite prevalent in many Charles x Erik fanfictions.
Often, there are themes related to their powers and how they interact with each other because of that. For example, how Erik's magnetic powers and Charles' telepathy play into their relationship. There are also themes about redemption, like Erik trying to make up for his past actions while being supported by Charles. And sometimes, the theme of them teaming up against a common enemy in a way that deepens their bond.
One common theme is the exploration of their relationship under new circumstances. The mpreg forces them to reevaluate their connection and face new responsibilities. Another theme is the protection of the unborn child. Erik often takes on a very protective role.
One main theme is the discovery of self. Without their mutant powers as the main focus, Charles and Erik have to look deeper into who they are as individuals. For example, they might question their values and beliefs in a more normal world setting.
In fanfiction, Erik's possessiveness of Charles might be an exaggeration of his character trait. He may have a sense of ownership over Charles due to their shared experiences, like founding the X - Men together. His possessive nature could also be a way to show his intense love or loyalty towards Charles, in a rather extreme way. He might think that Charles belongs with him more than with anyone else, whether it's in their mutant - related activities or in their personal relationship.
I also like 'The Unspoken Bond'. This fanfiction focuses on the unspoken emotions between them. It shows their relationship evolving in a more subtle way, through small gestures and glances. It's a very emotional read that really captures the essence of their connection.
One common theme is the struggle between their different beliefs. Charles often advocates for peaceful co - existence with humans while Erik believes in a more militant approach to protect mutants. Fanfics usually explore how they deal with this fundamental difference.
You can try searching on fanfiction websites like Archive of Our Own (AO3). It has a large collection of fanfics, and you can easily filter for Erik x Charles ones. Just use the search bar and type in relevant tags.
A fanfiction might start with Charles being seriously injured in an unexpected attack. Erik, who has always had a strong connection with Charles, is enraged at first. But then he realizes that he needs to be calm to help Charles. He takes Charles to a safe place and tries to find a way to heal him. Along the way, they face many challenges, both external threats and internal emotional struggles. As time passes, their bond becomes even stronger through this difficult experience.
I've heard of 'Unexpected Bond'. In this fanfic, the mpreg aspect adds a whole new layer to their relationship. It has some really touching moments as they navigate through the changes. There are also some interesting plot twists that keep you hooked. It's a great read if you like exploring different dynamics in their relationship.