One concept could be an OC who is a time - traveler and has to interact with Hirako Shinji at different points in his life. This would add an element of mystery and could change the course of the original story in unexpected ways.
One possible plotline could be that Shinji discovers a female character he cares about is pregnant, and he has to deal with the shock and then step up to support her. Maybe he has to face some challenges within his group or community because of the unexpected pregnancy.
Start with a distinct personality trait. For example, if you want your OC to be the opposite of Shinji in some ways, make them extremely confident and outgoing. This contrast will create interesting dynamics between the two characters.
There may not be a large number of Shinji Hirako fanfictions with a pregnancy plot specifically. However, if you join some Bleach - related fan communities (since Shinji Hirako is from Bleach), you might be able to ask fellow fans if they know of any such stories. They could point you in the right direction or even share some unpublished works they are aware of.
He could meet his future self. His future self might be wiser, having learned from all the experiences in the past. They could have a deep conversation about Shinji's purpose and the mistakes he's made.
Their relationship could be a bit rocky at first. They might argue about how to raise the baby. Asuka might be more strict while Shinji is more lenient. But through these disagreements, they gradually learn to compromise and in the process, their relationship deepens. They realize that they need to be united for the sake of their child.
In a novel, a character like Shinji Aoba could hold multiple significance. He might be a symbol of a certain ideology or social group within the fictional world of the novel. For example, if the novel is set in a post - apocalyptic world, he could represent hope or the struggle for survival. His relationships with other characters also play a big role in determining his significance. If he has a complex relationship with a villain or a love interest, it can add depth to the story.