We don't know exactly when the Akatsuki no Yona manga will come to an end. It all depends on the story's complexity and the author's vision. Maybe there are still many exciting chapters to come before the conclusion.
One idea for an Akatsuki no Yona pregnancy fanfic could be exploring how Yona copes with the physical and emotional changes during pregnancy. She's a strong character, so seeing her deal with the new challenges while still being a leader would be interesting.
In Akatsuki no Yona fanfiction, male OCs can add new dynamics. They might be designed as new allies for Yona and her group. For example, a male OC could be a skilled warrior from a far - off land who joins them on their journey, bringing with him unique fighting styles and cultural knowledge.
I'm not sure specifically what the 'shin jae ha girls love story' is about as it could be a very specific or personal story. It might be about the love experiences of girls related to someone named Shin Jae Ha.
The main characters in the 'shin jae ha girls love story' are most likely the girls themselves. There might also be other supporting characters such as friends, family members, or potential love interests. But without more information, it's all speculation.
To start writing an Akatsuki no Yona pregnancy fanfic, first, study the characters well. Know their personalities, relationships, and how they would react in this new situation.