In this fanfiction, perhaps there's a prophecy involved that ties their fates together in this strange way. The story could show how they try to protect the child from those who might want to harm it. There could be elements of mystery as they try to figure out the true nature of the child's magic, which might be a combination of their powers. Also, the relationship between Harry and Tom could transform from enmity to a more complex bond as they are about to become parents.
A third preference could be his excitement about becoming a father in the fanfiction. He could be seen shopping for baby clothes way in advance, daydreaming about what the baby will be like, and sharing his joy with his friends and family.
The characters would develop in many ways. Harry could become more cautious as he has someone else to think about now. He may also struggle with the new identity of being a pregnant person in a world that doesn't expect it from him. Tom might find himself in a position where he has to make sacrifices for the first time, which could change his whole view of power and control.
I'm not sure about specific 'Harry pregnant by Tom' fanfics as this is a rather unusual and often not canon - based concept. However, you can try searching on popular fanfic platforms like Archive of Our Own (AO3) where you might find such works if they exist.
Some fanfictions might explore the idea of the child's future. Since it's Harry and Tom's child, there could be speculations about the child's power. Will it be a force for good or evil? And how will Harry raise the child in a world that might be hostile to the idea of their union? There could also be a plotline where the pregnancy forces Harry and Tom to confront their own pasts and the history between them.
There's also 'Harry and Tom: The Forbidden Alliance'. It's a fanfiction that delves into a rather complex relationship between the two. They find themselves in a situation where they have to form an alliance against a common, dark enemy, and the story is filled with intense magical battles and emotional turmoil.
Harry might develop a stronger sense of justice due to his Gryffindor influence, while Tom could be more influenced by Slytherin's ambition. So Harry focuses on helping others, and Tom on achieving his own goals.
Tom, on the other hand, could start to see the value in friendship and loyalty through working with Harry. His character could soften a bit, realizing that there are other ways to gain power and respect than through fear and intimidation. For example, he could learn from Harry's ability to inspire people. They could also have a sort of mentor - mentee relationship in some aspects, with each teaching the other different spells or magical theories. This would make Tom a more complex and well - rounded character.
They could start by having small moments of understanding. For example, when one of them is in trouble, the other shows unexpected kindness. This could gradually build trust between them.