Perhaps there could be a rival who tries to break them apart. This rival could be someone from Sasuke's past who has a grudge against him and sees Sakura as a way to get revenge. They might spread false rumors about Sasuke to make Sakura question his loyalty.
One possible plot twist could be that Sasuke is not the real kidnapper. Maybe he has an evil clone or someone is framing him. Another could be that Sakura has a hidden power that she discovers during the kidnapping ordeal, which changes the power dynamic. For example, she might be able to communicate with a spirit that helps her escape.
One possible plot twist could be that an old enemy resurfaces just as they are about to get engaged. This enemy might try to disrupt their relationship by spreading false rumors about Naruto, like he has some hidden dark power that could endanger Hinata. Another twist could be that Hinata's family has some ancient clan secret that surfaces and creates a rift between them and Naruto's side. Maybe the secret is related to a power that the Hyuga clan once had and lost, and they fear Naruto might be involved in its re - emergence in a negative way.
Maybe there's a plot twist where Hermione discovers that Blaise has a hidden past or a secret mission that he hasn't told her about. This could put a strain on their engagement as she questions whether she really knows him. Or perhaps there's a magical curse that gets in the way of their relationship, and they have to find a way to break it in order to have a successful engagement.
Sakura and Shikamaru's Engagement. One sunny day in Konoha, Sakura was going about her usual business in the hospital when Shikamaru showed up. He seemed a bit more nervous than usual. After some stuttering, he finally pulled out a simple yet elegant ring and asked Sakura to marry him. Sakura, with tears of joy in her eyes, said yes.
Another plot twist could be related to the pregnancy itself. What if Sakura's pregnancy was not a natural one but rather the result of some kind of experiment or curse? This could lead to a whole adventure where she has to find a way to break the curse and save her unborn child. She might have to team up with unlikely allies and face powerful enemies in the process.
In a fanfiction like this, they could be on a diplomatic mission to another village. After the official business is done, they are invited to a banquet where they get drunk. While drunk, Sasuke might get into a friendly argument with Sakura about different fighting techniques. Sakura, being tipsy, could be more assertive in her views. This could lead to them sparring drunk (in a harmless way) and then realizing how much they enjoy each other's company even when not in a serious ninja - related situation.
In some fanfictions, their marriage could be a political move at first. Maybe the Hidden Leaf Village needs to strengthen its alliances, and a union between Sakura, a strong kunoichi, and Sasuke, a powerful shinobi, seems like a good idea. But as they spend time together, they start to develop real feelings for each other. They have to deal with the gossip and doubts of the villagers, but in the end, they prove that their love is real.
One possible plot twist could be that Sasuke has been under a genjutsu that makes him target Sakura, and Naruto has to break the genjutsu first to truly save her. Maybe the one who cast the genjutsu is a new, powerful enemy who wants to disrupt the bond between Team 7. Naruto, with his strong will and newfound powers, manages to find the source of the genjutsu and free Sasuke from its control, thus saving Sakura from harm.
Another plot twist might be that Sasuke discovers a hidden enemy who is targeting their unborn child. This could lead to Sasuke having to go on a dangerous mission while Sakura is pregnant to protect their family. It would add a lot of tension to the story as Sakura is left at home worried about him.
Imagine a fanfiction where Sakura, instead of being in love with Sasuke, has feelings for Naruto all along. This would completely change the love triangle aspect and lead to a lot of interesting interactions between the three main characters. It could also cause Sasuke to re - evaluate his relationship with his teammates.