Howard and Bernadette would have a more experienced perspective. Bernadette would share her own pregnancy stories with Amy, both the good and the bad. Howard would probably make some corny jokes about the situation, but deep down be really happy for his friends. All the characters would come together to form a sort of support system for Amy and Sheldon during this big life change.
One plot idea could be that Amy and Sheldon have to deal with all the chaos that comes with preparing for a baby while still maintaining their unique relationship quirks. For example, Sheldon might try to create a scientific schedule for the baby's development and activities, much to Amy's amusement and occasional frustration.
Leonard would likely become even more protective of Penny. Their relationship would deepen as they face the new challenges of impending parenthood together. He might start to take on more domestic responsibilities, like making sure Penny has a healthy diet during her pregnancy.
Well, their relationship could become even closer. Sheldon might become more protective of Amy. He would be constantly worried about her well - being and that of the baby. For example, he could start monitoring her diet very strictly, which might annoy Amy at times but also show his care.
One common theme is the characters' reactions to the pregnancy. For example, if Penny were to get pregnant, Leonard's response would be a big part of the story. Another theme is how the pregnancy affects the group's dynamic. The friends might start acting differently towards the pregnant character.
One possible fanfiction could be about how Bernadette adjusts to her pregnancy while still trying to keep up with her demanding job. Maybe she has to deal with morning sickness at work and tries to hide it from her colleagues at first.
Another aspect could be Amy's own journey during pregnancy. She might struggle with some of the physical changes and hormonal mood swings. Penny could be there for her as a great friend, sharing her own experiences or just being a listening ear. Also, Raj and Howard could try to give their own unique, and probably comical, parenting advice based on their knowledge from movies and books.
Rory would be a mix of emotions. He'd be worried about Amy, but also feel a bit left out since she's having a baby with the Doctor. River might be more understanding, given her complex relationship with the Doctor. She could offer some advice based on her own experiences with time - travel and family. The other friends like Craig might be shocked but also try to support them in their own bumbling, human way.
She might become more responsible. Penny has always been a bit carefree, but pregnancy could force her to think about the future more. For example, she may start to save money or look for more stable acting jobs.
One popular 'Big Bang Theory' pregnant fanfiction might be those that explore Penny getting pregnant. Some fanfics could focus on how the group dynamic changes with a new addition on the way. However, it can be a bit hard to specifically recommend one as there are many out there. You can try searching on fanfiction - specific websites like Archive of Our Own.
There might be a fanfiction where Penny is pregnant and she starts to feel really isolated because she's the only one in the group going through this. She misses her old life a bit, and the story could be about how she overcomes these feelings with the support of her friends and Leonard. It's very emotional as it deals with her internal struggles.