One possible plotline could be Buffy's friends rallying together to rescue her. They might use their unique skills, like Willow's magic, to track down Angelus and his lair.
Perhaps Buffy manages to outwit Angelus while being kidnapped. She could play mind games with him, using her knowledge of his past and his weaknesses. For example, she might remind him of his love for her when he was Angel, which could cause internal conflict for him.
The Scooby Gang, including Willow, Xander, and Giles, would surely be key characters. Giles, with his vast knowledge, would be trying to figure out a way to save Buffy. Xander would probably be the one ready to jump into action, and Willow could use her magic to try and locate Buffy or counter any dark magic Angelus might be using.
One possible motive could be revenge. If Buffy has previously defeated him or his plans, he might want to turn her into a vampire as a form of ultimate revenge, to make her into the very thing she has been fighting against.
Popular themes often include redemption arcs. If Angelus were human, how would he seek redemption for all the wrong he did as a vampire? Also, love and friendship are big themes. Fans like to write about how Buffy and Angelus could have a more 'normal' love story as humans, or how their friendship could develop in a different way. There are also themes related to the challenges they face in a human world, like dealing with mortality, which is very different from their vampire - filled existence.
One possible plotline could be that Cato kidnaps Katniss as a strategic move in the Hunger Games. Maybe he thinks he can use her to draw out other tributes or get an advantage over the Careers. Another could be that Cato has some sort of personal vendetta against Katniss that leads him to kidnap her outside of the normal rules of the Games.
One possible plotline could be Bella's friends and the Cullens coming together to rescue her. They might plan a stealthy infiltration of Aro's lair, using their unique abilities. For example, Alice could use her visions to anticipate Aro's moves and find the safest way in.
Perhaps the rogue is hired by an evil force to kidnap Lucy. She could be the daughter of an important figure or a person with unique abilities that the evil force wants to use against the good side. As the story progresses, Lucy could try to escape or even try to turn the rogue to her side by appealing to his better nature.
Another plotline might involve Tom believing that Harry has some hidden power that he can extract or use for his own dark purposes. So, he kidnaps Harry to figure out how to access that power. This could lead to a series of attempts by Tom to break Harry's will and get him to reveal the secret. For example, he might put Harry in a magically sealed room and use various spells to try and force the information out of him. Harry, on the other hand, could be trying to resist and find a way to escape or signal for help.
Well, it could be that Lucy has a misunderstanding. Maybe she overhears something that makes her think Natsu is planning to leave the guild without saying goodbye. So, in a fit of panic and anger, she kidnaps him to get an explanation. Then, as the story progresses, they have a heart - to - heart conversation where the misunderstanding is cleared up. It could also be a situation where Lucy is competing with someone else for Natsu's attention, and she thinks that by kidnapping him, she can have him all to herself for a while and figure out how to make him like her more.
Love and sacrifice also play a big role. There could be a situation where Angelus has to make difficult choices to protect Buffy and the unborn child, or Buffy might have to sacrifice her slayer duties to some extent for the sake of the pregnancy. These themes of love, protection, and sacrifice are often central in such fanfictions.
Common themes include the duality of their natures. Buffy is a hero, and Angelus is a villain, yet they are drawn to each other. The theme of secrecy also pops up often. Their relationship can't be out in the open easily. Lust is a very obvious theme considering the 'lemon' nature of the fanfiction. But there are also themes of sacrifice, where either Buffy or Angelus might have to sacrifice something for the other or for the greater good in the context of their relationship.