Well, a major plot twist could be that Bulma's inventions start to malfunction in a way that affects Naruto's ninja abilities. Maybe a device she creates to enhance chakra backfires and starts to drain it instead. This would cause a rift in their relationship as they struggle to figure out what went wrong. Another twist could be that a powerful being from another dimension arrives and wants Bulma as his bride instead of Naruto. Naruto would then have to fight this being not only to protect Bulma but also their relationship and the two worlds they are from.
A really interesting plot twist might be that Naruto discovers he has some sort of hidden connection to the Dragon Ball universe through his family line. This revelation could change the dynamic of his relationship with Bulma. Or perhaps Bulma accidentally creates a time - traveling device and they end up in a completely different era in either the Naruto or Dragon Ball timeline. This could lead to all sorts of new challenges and opportunities for their relationship, as they have to adapt to the new situation while still maintaining their love for each other.
A possible plot twist could be that Gohan discovers a hidden power within himself because of his relationship with Bulma. Maybe her scientific experiments accidentally trigger something in his Saiyan genes. Another twist could be that their marriage is part of a bigger plan by an enemy to disrupt the Z - fighters. The enemy might think that by causing this strange union, they can create chaos among the group.
One possible plot twist could be that Anko is actually working undercover for an enemy organization all along. She got close to Naruto to gather information, but then truly fell in love with him during their relationship. Naruto would be devastated when he finds out and have to decide whether to trust her again or not.
A possible plot twist could be that Naruto discovers he has a hidden connection to Smaug. Maybe he is related to Smaug in some long - lost magical way. Another twist could be that Smaug is not really a villain in this fanfiction. Instead, he is being controlled by a greater evil, and Naruto has to figure out how to free Smaug from this control. It would change the whole dynamic of their relationship from one of enmity to a more complex situation where they could potentially become allies.
A possible plot twist could be that Naruto turns out to be a keyblade wielder in disguise. This would completely change the dynamic as everyone in the Naruto world and Kairi would be shocked. It could also mean that Naruto has a whole different origin and mission related to the world of Kingdom Hearts.
A possible plot twist could be that one of the Autobots is actually being controlled by an old enemy from Naruto's world. This enemy has found a way to hack into the Autobot's system and use it for their own revenge. So, while the Autobots seem to be helping Naruto, in reality, they might be leading him into a trap.
One possible plot twist could be that Konan discovers a hidden power within Naruto that he's not even aware of himself. This newfound power could attract the wrong kind of attention from other ninja villages or even from some secret organization. Another twist could be that Konan's past catches up with her in a way that endangers their marriage. Maybe an old enemy resurfaces and tries to use Konan to get to Naruto. Or perhaps there's a prophecy that says their union will bring either great destruction or great peace, and they have to figure out which it will be.
One possible plot twist could be that Obito was actually being controlled by a more powerful force all along. When his identity is revealed, it seems like he's the mastermind, but later it's discovered that he was just a puppet. Another twist could be that Obito has a secret alliance with a character that no one expected, like a member of the Leaf Village elders. This alliance was formed for a reason that is only revealed later in the story.
Another plot twist could be that Naruto's own chakra has an unexpected effect on the vampire. Perhaps it changes the vampire in some way, either making them more powerful or causing some sort of internal struggle within them. This would then have a big impact on their relationship as they both have to figure out how to deal with this new situation.
Perhaps Naruto discovers that Kisame has a family he's been protecting in secret, and this new knowledge makes Naruto see Kisame in a different light. He then tries to help Kisame reunite with his family while also dealing with the Akatsuki.
Perhaps the Akatsuki's neglect is a part of a much larger plan by Naruto himself. He might be trying to lull the Akatsuki into a false sense of security while he's actually gathering information or power in the background. Then, when the Akatsuki least expects it, Naruto reveals his true intentions and uses the situation to his advantage, which would be a huge plot twist for the readers who thought Naruto was just being mean or indifferent.