Maybe the end would see them sacrificing their love for the greater good. They realize that their relationship is causing too much chaos in the supernatural world, and in order to save their friends and the town, they part ways. But they always carry the memories of their love with them, and in a way, it makes them stronger individuals in their respective worlds.
The relationship would shock Damon and Stefan. They have always been protective of Elena, and Kol has a reputation as a dangerous vampire. Damon might be especially angry and try to drive Kol away from Elena. Bonnie would also be concerned as she has seen Kol's past actions. However, Caroline might be more open - minded and could potentially try to help Elena understand her feelings for Kol. Klaus would be an interesting factor too. He might see Kol's relationship with Elena as a way to gain an advantage in his power games or he could be protective of his brother's new - found love.
One possible plot could be that Kol, being the charming and somewhat dangerous vampire he is, initially catches Elena's eye in a moment of unexpected encounter. Maybe at a supernatural gathering. He starts to show her a different side of the vampire world, full of ancient secrets and forgotten powers. This new perspective intrigues Elena, and as they spend more time together, they develop feelings for each other despite the many obstacles like the existing relationships Elena has and the disapproval from other vampires and her friends.
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.
Caroline would probably be very shocked at first. She might have some concerns about Kol's intentions, but seeing how much he loves Elena and how excited they are about the baby, she would gradually come around. Stefan would be more reserved in his reaction. He would be worried about Elena getting hurt, but he would also respect her decision. Damon, on the other hand, would be more vocal. He might make some snide remarks at first, but deep down he would also want Elena to be happy. Bonnie would be practical. She would offer to use her magic to make sure Elena has a safe pregnancy and that the baby is healthy.
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.
The Salvatore brothers might be a bit protective at first. Damon would probably be skeptical about Kol being in the picture, but over time he'd warm up to the idea for Elena's sake. Stefan would be more accepting right away and offer his support. Bonnie would be excited and use her magic to make sure the baby is safe. Caroline would be all over helping with baby clothes and advice. As for the Mikaelson family, Klaus might see the baby as a way to strengthen his family's hold in the supernatural world, but also be protective in his own way.
To develop their relationship realistically, they need to start as friends. Kol could show Elena his more vulnerable side, like his past traumas as an Original. Elena, in turn, can share her own struggles with him. They should have conversations about their different views on life, death, and immortality. Then, slowly, feelings of attraction can start to develop. For example, Kol might save Elena from a dangerous situation, and she realizes she has more than just gratitude for him.
One possible plot could be that Kol and Elena meet by chance in a place outside of Mystic Falls. They initially clash due to their different personalities and pasts. Kol is intrigued by Elena's tenacity and her kind heart despite all she has been through. As they are forced to spend more time together, perhaps because of a common threat, they start to understand each other better. Elena begins to see Kol not just as one of the bad guys but as someone with his own complex story. Their love could develop gradually as they share their secrets and fears, and finally, they realize they are in love.
A great fanfiction could be 'Elena and Kol's Eternal Love'. This might show their relationship as something that endures through various trials and tribulations. It could start with their initial attraction, develop into a deep love, and then face obstacles like the disapproval of other characters or the forces of the supernatural world that they inhabit. Their love would be the central theme, with the story exploring how they overcome difficulties to be together.
In fanfiction, Kol and Elena's love often brings a unique dynamic. It can show Kol's more complex and passionate side. Their relationship might be explored in different settings, like in a modern AU where they meet in a different context than in the show. It allows for creative interpretations of their personalities and how they interact.