The other vampire clans might be antagonists. They could see the baby as a threat to the vampire hierarchy or fear that the baby will have powers that could disrupt the balance among vampires. For example, the Originals' siblings who don't trust Kol might try to stop Elena from having the baby.
One possible plot twist could be that the pregnancy is part of a magical spell gone wrong. Maybe a witch with a vendetta against either Kol or Elena cast a spell that made Elena pregnant, and now they have to figure out how to reverse it without harming the baby or themselves. Another twist could be that the baby has some sort of special power that attracts all kinds of supernatural threats, forcing Kol and Elena to constantly be on the run and protect their unborn child.
Their friends might be shocked at first. Damon, for example, might be a bit jealous as he had feelings for Elena in the past. But he would also probably be protective. Bonnie would likely use her magic to ensure the pregnancy goes smoothly and help protect Elena from any supernatural threats. Stefan would be supportive, seeing it as a new chapter in Elena's life.
A plot could involve Elena and Kol having to deal with an ancient prophecy related to their unborn child. They have to figure out what it means and how to prevent any negative consequences. Along the way, they learn more about each other's pasts and how their families' histories are intertwined with this prophecy, which makes them more determined to protect their family.
They could develop a shared hobby. For instance, they both get into painting and use it as a way to express their emotions and connect on a deeper level. It would be unique because it shows a side of them that's not related to their supernatural natures.
In many 'The Vampire Diaries' Kol and Elena fanfictions, their relationship can be quite complex. Kol is often depicted as a wildcard, and his interactions with Elena can range from intense and dangerous to oddly alluring. Some fanfictions might explore a forbidden - love - type scenario, where Kol's vampiric nature and his family's history with Elena's friends create barriers. However, they also might find common ground in their mutual experiences of being caught up in the supernatural chaos of Mystic Falls.
One possible plot could be that Kol kidnaps Elena to use her as bait for his enemies. He might think that by having Elena in his custody, her friends, especially the Salvatores, would come to rescue her and he could then ambush them. Another idea is that Kol has some sort of vendetta against Elena because of her connection to the doppelganger bloodline. So he kidnaps her to extract revenge on the entire line.
One common theme is forbidden love. Since Kol and Elena might be from different 'worlds' or groups within the show's lore, their relationship can be seen as something that goes against the norm. Another theme could be passion. The kiss is often a moment of intense passion that drives the story forward.
Another character that could be important is Elijah. He is often the voice of reason among the Originals. He might try to guide Kol in his relationship with Elena, or he could be against it if he thinks it will bring trouble to their family. So in this fanfiction, he could be a key figure in the development of the Kol - Elena relationship.
The plot might involve Kol falling in love with Elena. Kol, being the wildcard among the Originals, could try to win Elena over in his own unique way, which might create a lot of drama with the other characters like Damon and Stefan who are also interested in Elena.