Well, a plot twist could be that they find out they are related in some convoluted way. Maybe there's a long - lost family connection that was hidden. This would create a huge moral and emotional dilemma for them. Or, perhaps there's a magic or chakra - related twist. For instance, a curse is placed on their relationship that makes them forget each other every few days, and they have to constantly fall in love all over again. It would add an interesting layer to their married life as they struggle to find a way to break the curse while also building a life together.
One possible plot twist could be that Anko is initially hesitant about the marriage due to her past traumas. Maybe she has some secrets she's afraid to share with Kakashi. For example, her past association with Orochimaru might make her doubt her own worthiness in a relationship. Another plot twist could be external threats to their marriage. The shinobi world is full of danger, and perhaps an enemy targets Anko to get to Kakashi, or vice versa. This could put a strain on their relationship as they try to protect each other while also dealing with the normal stresses of a new marriage.
A plot twist could be that Kakashi is forced to go on a long - term mission right before the wedding. Anko, feeling left behind, has to re - evaluate her feelings for him. She might start to question if she can handle a life where Kakashi is often away. On the other hand, when Kakashi returns, he might find that Anko has changed in some ways, and they have to work hard to rekindle their relationship. Also, there could be a plot twist where a prophecy or an old shinobi law comes into play that challenges their marriage, like a requirement for Kakashi to marry someone from a certain clan for political reasons.