I'm not sure there are a lot of positive 'romance' scenarios where one character kidnaps the other in this context. However, in fanfiction, it could be a really twisted take on their relationship. Maybe the writer is exploring a darker side of Daryl's character where he feels compelled to kidnap Rick for some reason like he thinks it's the only way to protect him from a greater danger that Rick doesn't see. But this is quite a departure from their typical relationship in the show.
Obviously, the two Daryls are the main characters. They are the ones at the center of the story, especially since the title implies that they are the ones who are pregnant.
One possible plot could be the two Daryls navigating the challenges of pregnancy together, like dealing with morning sickness and preparing for the baby's arrival. They might also face judgment from others in their community.