One great Daryl x Negan fanfiction could be 'The Unlikely Bond'. It explores how their initial enmity gradually turns into an understanding. Another is 'Beyond the Rivalry' which delves deep into their complex relationship.
Fanfiction.net is also a good place to look for Daryl x Negan fanfiction. There are thousands of stories there. However, the quality may vary, so you might need to read a few to find the ones that really suit your taste. You can also check out some The Walking Dead - themed fanfiction forums where fans might share their own works or recommend good Daryl x Negan stories.
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.
Well, Negan first appears in the series as a major antagonist. He has a very distinct personality. He's confident, often to the point of being cocky. His group, the Saviors, would extort other communities for resources. He uses fear as a tool to control those around him. His actions have a huge impact on the other characters in the show, like Rick and his group.
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.