It could also be that Beth's pregnancy is a result of an alien encounter. The alien species could have some sort of ulterior motive for impregnating her. This would then lead to the family having to deal with the alien race while also dealing with the impending birth. They might have to fight off the aliens who are trying to take the baby for their own purposes.
Perhaps the pregnancy could lead to a whole new adventure for the family. The baby could have some sort of special ability due to the family's unique circumstances. For example, it could be immune to the effects of some of Rick's wacky inventions. This would then cause the family to have to protect the baby from those who might want to use its power for their own gain.
One possible storyline could be that Morty gets hurt during one of their inter - dimensional adventures. Maybe they land on a planet with dangerous wildlife and Morty is attacked while Rick is distracted trying to fix the portal gun. Another could be that Morty gets injured in a fight with one of Rick's enemies who target Morty to get back at Rick. It could also be that Morty gets hurt due to his own clumsiness while handling some of Rick's wacky inventions in the garage.
In this fanfiction, perhaps Amy is in a difficult situation when she gets pregnant. For example, she could be in the middle of pursuing a big career opportunity. The story might revolve around her decision - whether to keep the baby and put her career on hold or to choose her career over the pregnancy. There would be a lot of internal struggle and emotional turmoil.
Some writers might create this type of fanfiction for shock value. They know that the original relationship between Rick and Morty has a certain dynamic, and by making it into an abusive one, they can get a strong reaction from readers. Also, it could be a form of expressing their own dark interpretations of power dynamics within the show.
Summer might be jealous at first. She's used to being the only daughter and having a new baby in the family could mean less attention for her. But as she realizes that the baby is going to be part of the family, she could become really protective. She might also start to think about how she can be a cool big sister and teach the baby all sorts of things she's learned from her adventures with Rick and Morty.
One possible plotline could be Summer dealing with the reactions of her family. Rick might be initially dismissive but then secretly help her. Morty could be in shock but try to support his sister. For example, they could go on adventures to find a solution to her pregnancy in different dimensions.
One possible storyline could be that Rory and Dean face unexpected challenges. They might struggle with how to balance their young lives with impending parenthood. Maybe they have to deal with the reactions of their families and friends, some being supportive and others doubting their ability to handle it. Their relationship could be put to the test as they navigate through the pregnancy, with all the hormonal changes and financial concerns that come with it.
It could be some alien she met during one of Rick's adventures. Maybe she got involved with a being from a different dimension who had a human - like form and they had a brief encounter that led to the pregnancy.
A storyline could revolve around Beth's pregnancy and how it changes her perspective on life. She has been so tough and involved in the cut - throat world of the ranch. But now, with a baby on the way, she might start to consider a more peaceful future. She could also start to mend fences with some of the people she has had conflicts with in the past, like her family members.
One of the best storylines is when Rick and Morty go to the Purge planet. It's a really interesting concept as they have to survive in a world where all crime is legal for one day. Morty is scared but Rick is kind of excited to see what chaos he can cause or avoid. They meet all sorts of strange characters during that time.
There might be a fanfiction where Rick discovers that an old acquaintance is pregnant and he has to deal with the moral and ethical implications of it. Since Rick is often amoral, this could lead to a really interesting story arc as he tries to figure out what to do. For example, he might be tempted to use the pregnancy for some sort of scientific experiment but then has a change of heart.