Maybe a human who has discovered the existence of vampires and wants to use Renesmee to prove it to the world. They might think that by kidnapping her, they can expose the vampire world and gain fame or fortune. Another option is a rogue werewolf who doesn't like the alliance between the vampires and werewolves and decides to take Renesmee as a way to break that bond.
I'm not sure if there is a specific 'Jacob gets Renesmee pregnant' fanfiction. Fanfiction can cover a vast range of ideas, but this is a rather controversial and unusual plot. It might exist in some corners of the fanfiction world, but it also goes against some of the original concepts in the source material. You would need to search through various fanfiction platforms like Archive of Our Own or FanFiction.net to find out for sure.
The Cullens are the most likely. Edward, with his mind - reading ability, can try to sense where Renesmee is. Bella, with her strong will and vampire strength, would be at the forefront of the rescue mission. And Carlisle, with his wisdom and leadership, would plan the whole operation.
Edward and Bella could be important characters. As Renesmee's parents, they would likely be deeply involved in the situation. They might be worried about her well - being, try to protect her from any threats related to the pregnancy, and offer their support and advice.
In such a fanfiction scenario, first of all, the Volturi would likely have their reasons for taking Renesmee, perhaps believing she is a threat to the vampire world order. Once taken, the Cullens would be desperate. Alice might use her visions to try to find Renesmee's location. Jacob could also play a crucial role as he has a special bond with Renesmee. He may attempt to lead a rescue mission or at least try to communicate with her while she is in the Volturi's custody. The story would be filled with drama, action, and the exploration of relationships between different characters in the 'Twilight' universe.
In many fanfictions where Gil gets kidnapped, it could be for various reasons. Maybe it's by a rival group who wants to use him as a bargaining chip against his friends. For example, in some stories, Uma's crew might be targeted because they are seen as a threat in the Isle of the Lost. Gil, being a part of that crew, gets kidnapped. Writers often use this plot device to create drama and show the loyalty and courage of the other characters as they try to rescue him.
One interesting Renesmee kidnapped fanfiction could be where the Cullens have to use all their vampire powers and alliances to track her down. It might involve a new vampire clan that has some grudge against the Cullens and decides to take Renesmee as a way to get back at them.
The implications would be significant. First, it would change the entire perception of the characters. Jacob and Renesmee's relationship has certain boundaries in the original. This fanfiction concept would break those boundaries and create a whole new set of relationships and family dynamics. It might also face a lot of criticism from the fan community for straying too far from the source material. Additionally, it could raise ethical questions about the nature of the relationship depicted in the fanfiction, as Renesmee has a unique origin in the original story.
Another plot twist could be that the pregnancy progresses at an extremely abnormal rate. This could put Renesmee's life in danger and force her and her loved ones to find a way to slow it down or deal with the consequences of a very rapid development. It could also lead to some unexpected revelations about her own nature and abilities.