Perhaps there's a spell that goes wrong and affects Hermione's pregnancy in an unexpected way. Maybe it makes the baby grow at an accelerated rate for a short time, causing panic among them until they find a way to reverse it. This would add a lot of drama to the story.
A plot twist could be that Hermione discovers she's having twins instead of one baby. This would double the excitement and also the stress for them. Ron would have to scramble to get twice as many baby supplies.
A really interesting plot twist could be that Hermione gets pregnant, but then Ron suddenly loses his memory. So he has no idea that they are in a relationship or that she's carrying his child. Hermione then has to not only deal with the pregnancy but also try to help Ron regain his memory. Another plot twist could be that they find out that the baby is part - Veela, which is completely unexpected considering their family backgrounds. This would bring in a whole new set of challenges and cultural differences to deal with. And yet another could be that the Ministry of Magic tries to interfere with their pregnancy for some political reason, creating a conflict between them and the wizarding government.
Well, one possible plot twist could be that the father of Hermione's baby is not who everyone expects. Maybe it's a character who has been secretly in love with her all along. Another could be that there are some magical complications during the pregnancy that put Hermione's life at risk. And the baby could have some extraordinary magical powers even before it's born.
One possible plot twist could be that Ron initially panics about the pregnancy but then realizes he has strong feelings for Shego. Maybe he tries to keep it a secret from their respective friends and enemies, which would lead to a lot of sneaking around and near - misses of being discovered.
Perhaps Snape and Hermione were secretly working on a time - traveling potion project. Due to some unforeseen consequences of time manipulation, Hermione ended up pregnant. It could also be that a powerful magical artifact they were studying had some sort of fertility - enhancing effect that they didn't anticipate at first.
Another plot twist could be that Lucius's act of saving Hermione causes a rift between Hermione and Ron that is hard to repair. Ron might misunderstand Hermione's connection with Lucius and think she is now on the 'dark side' so to speak. This could lead to a complex love triangle situation where Hermione has to prove her loyalty to her friends while also dealing with her gratitude towards Lucius.
Well, perhaps in the fanfiction, Ron and Hermione discover that Rose has some unique magical abilities even before she is born. This causes a stir in the wizarding world, and they have to deal with unwanted attention from the Ministry of Magic. At the same time, they are also trying to balance their normal lives, like Ron still working at Weasley's Wizard Wheezes and Hermione doing research at the Ministry. They have to find a way to protect their family while also living their lives.
In this fanfiction, a great plot could be that Ron and Hermione discover some ancient magic related to their pregnancy that has ties to Hogwarts' history. As they explore this, they learn more about themselves and their relationship. Also, they could struggle with how to tell their families, especially since they're still at school. Ron might be worried about his mother's reaction, while Hermione might be concerned about how her Muggle parents will take the news. They'd have to navigate all these emotions while still being students at Hogwarts.
One possible plot twist could be that they face unexpected magical difficulties. For example, a spell gone wrong might temporarily affect their chances of having a baby. Maybe a dark magic remnant in their house, left over from their previous battles, starts to interfere with the natural process. Another twist could be that they discover a long - lost family curse that might be passed on to their potential child. This could lead them on a journey to find a cure before they actually conceive.
Well, perhaps Bonnie finds out during her pregnancy that Ron has been keeping a big secret from his past, like a previous relationship that he never fully disclosed. This could lead to a lot of drama and conflict between them. They would have to work through it while also preparing for the new baby.
One interesting plot point could be an alternate take on their relationship during the Horcrux hunt. Maybe in the fanfic, they face a particularly difficult Horcrux that tests their bond in a new way, like a Horcrux that plays on their insecurities about each other.