One idea could be that the baby has some sort of special power. Since Elijah is a powerful vampire and Katherine has her own unique charm and skills, their baby might inherit something extraordinary. Maybe the baby can control the minds of vampires or has an uncanny ability to sense danger.
One idea could be that they face challenges from the supernatural community due to their unborn child's potential power. The other vampires or witches might see the baby as a threat or a valuable asset. Another plot could be them preparing for the arrival in a world full of danger, like building a safe haven and gathering allies.
In Klaus and Katherine baby fanfiction, it could be interesting if the baby was born with a prophecy attached to it. The prophecy might say that the baby will either bring great peace or great destruction to the vampire world. This would put a lot of pressure on Klaus and Katherine to ensure the baby grows up to be a force for good. The baby might also have an unusual appearance, a blend of Klaus and Katherine's features, which could make it stand out among other supernaturals. And as the baby develops, it could start to show signs of having a moral compass that is different from both its parents, leading to some interesting family dynamics.
One idea could be that Elijah discovers a lost baby and decides to protect it at all costs. He might use his skills and resources to keep the baby safe from various threats in a fantastical world.
The Originals' enemies, like the werewolves, could be antagonists. They might see the baby as a symbol of the vampires' growth and power and try to harm it. Also, some of the other vampire factions who are against the Mikaelsons could target the baby to get back at Elijah and Katherine.
Well, it could be about how they deal with the unexpected pregnancy. Maybe they struggle to accept it at first, then gradually come to terms and start preparing for the baby.
One interesting fanfiction could be about Elijah trying to redeem Katherine. Maybe he sees a side of her that others don't, and he attempts to guide her towards a more positive path. It could involve them traveling together and facing various challenges from the vampire world.
Perhaps in the fanfiction, Bella and Katherine are related through a shared ancestor who was a powerful witch. This connection could lead to a plot where they both have to team up to face a common enemy. The enemy could be a group of rogue vampires who are after a magical artifact that was once in their family's possession. Bella, with her newfound knowledge of her relation to Katherine, has to navigate the dangerous vampire world she's already a part of, while also dealing with Katherine's more self - serving nature.
A possible plot could be that Bella and Elijah are from different species in the fanfiction world, for example, one is a vampire and the other a werewolf (assuming such a scenario exists in this fanfic). Their pregnancy is then a very unique situation that defies the norms of their world. This could lead to a lot of drama, with different factions either trying to protect or harm them because of the baby. They would have to rely on each other and a few trusted friends to navigate this new and dangerous situation.
One idea could be that Elijah discovers he is going to be a father during a time of great turmoil in the supernatural world. His family's enemies might see this as a weakness and target him and his unborn child. Another plot could involve Elijah trying to protect the mother of his child from the dangerous vampire politics and ancient curses that could harm her or the baby. Maybe he has to make a deal with a powerful witch to ensure a safe pregnancy for the mother.
One popular story might be 'Eternal Love of Elijah and Katherine'. It could explore their relationship in a different timeline, perhaps where they meet under different circumstances and their love blossoms in a more peaceful setting. Another could be 'Katherine's Redemption through Elijah's Love', which may focus on Katherine's character development as she is loved by Elijah.