The Salvatores, Damon and Stefan, are likely candidates. They have always been protective of Elena and would do anything to save her. They could use their vampire skills and knowledge of Kol's weaknesses to stage a rescue.
Caroline might play a role in saving Elena too. She's strong, both physically as a vampire and also in terms of her determination. She could team up with the others, like Tyler if he's in the picture, and they could all work together to rescue Elena from Kol. They could come up with a clever plan that involves using Kol's overconfidence against him.
Bonnie could also be the one to save Elena. As a powerful witch, she could use her magic to track Kol and Elena down. She might even be able to conjure up some spells to defeat Kol and free Elena. And if Jeremy is still around, he could also help out, given his skills as a hunter.
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.
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.
Mario would be a likely candidate. He's always been the hero in the Mushroom Kingdom and has a lot of experience dealing with villains like Tatanga. He could use his jumping skills and power - ups to make his way to Tatanga's lair and rescue Rosalina.
The other Sailor Scouts would be the most likely ones to save her. They are her friends and comrades in the fight against evil.
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.
The Salvatore brothers, Stefan and Damon, are the most likely candidates. They both have a strong connection to Elena and would stop at nothing to rescue her. Stefan's sense of chivalry and Damon's fierce protectiveness would drive them to track down Katherine and get Elena back.
The main characters are clearly Kol and Bonnie. Kol is the one doing the kidnapping, and Bonnie is the victim of this action.
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.
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.
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.