One plot idea could be that Koga and Kagome accidentally end up in a situation where they have to take care of a baby together. Maybe they find the baby in the forest and have to figure out how to keep it safe while also dealing with their own feelings for each other. Another idea is that a magic spell causes them to have a baby of their own in a strange, otherworldly way. This would lead to a whole new set of challenges as they navigate being new parents in a world full of demons and magic.
A plot idea could be that Kagome's pregnancy gives her new powers or a stronger connection to the spiritual world. This could attract the attention of evil forces who want to use her unborn child for their own gain. Koga has to fight off these threats while also taking care of Kagome during her pregnancy. Another idea is that they go on a journey to find a special place or item that is said to ensure a safe delivery for their baby. This journey could be filled with challenges and revelations about their relationship.
One idea could be that the baby has some unique powers that are a combination of Kagome's spiritual powers and Sesshomaru's demon abilities. This could lead to various adventures as they try to protect the baby from those who might want to use its powers for evil.
One common theme could be family. Since it involves a baby, it might explore how Koga and Kagome adapt to being parents and creating a family unit.
Perhaps in the fanfiction, Koga kidnaps Kagome believing she has some special power that can help his wolf demon tribe. He might think that she can heal the sick among them or bring good fortune to their land. However, Kagome doesn't actually have this power, and she has to convince Koga to let her go while also trying not to anger him too much. As she's held captive, she might learn more about the wolf demons and their way of life, which could be an interesting aspect of the story.
I'm not sure of specific 'koga kagome baby fanfiction' stories off - hand. You could try searching on fanfiction websites like FanFiction.net or Archive of Our Own. Just type in the relevant keywords and see what comes up.
The story could be full of challenges. Koga, being a wolf demon, and Kagome, a human with spiritual powers, would face many difficulties in raising their baby. Maybe the baby would inherit unique powers from both parents, leading to some interesting adventures as they try to protect the baby from those who might want to use its powers for evil.
With a baby in the story, their relationship could become more solid. The shared responsibility of taking care of the baby might make them realize how much they rely on each other. For example, Koga could use his speed and strength to protect the baby from physical threats, while Kagome could use her knowledge and healing abilities. This mutual dependence could deepen their love for each other.
One common plot twist could be that Koga actually kidnapped Kagome to protect her from a greater threat. Maybe there's a powerful demon that's after Kagome, and Koga, despite his rough ways, believes he can keep her safe. Another twist might be that Kagome discovers she has some hidden power while in Koga's captivity that helps her either escape or gain an understanding of the demon world in a new way.
Well, a plot could involve Naraku being cursed to feel all the pain he has caused, and Kagome is the only one who can ease his suffering. So he has to follow her around. Or perhaps in a fanfiction, Naraku and Kagome get trapped in a magical realm where they have to rely on each other to survive. Also, a story could be about Naraku having a split personality, and one part of him is attracted to Kagome and tries to break free from his evil self to be with her.
One plot idea could be kagome getting involved in the vampire - werewolf conflict in the Twilight world. Maybe she has some unique powers from her own world that could change the balance. Another idea is that kagome and Bella become friends and kagome tries to help Bella deal with the supernatural chaos around her.