It could be that the zombies in Z Nation are actually the result of an experiment gone wrong by Orochimaru. He was trying to create some kind of immortality formula but it backfired and created the zombies. And now Naruto and his friends have to stop Orochimaru from making it worse.
Perhaps the Akatsuki from Naruto are actually behind the zombie outbreak in Z Nation. They wanted to create chaos and find some powerful artifacts in the Z Nation world. So they unleashed the virus as part of their grand plan.
A big plot twist could be that one of the main Naruto characters gets bitten by a zombie but instead of turning into one, their chakra somehow mutates the virus and they become a super - powered anti - zombie being. This would completely change the dynamics of the story.
One interesting plot twist could be a character traveling back in time and accidentally changing the events that led to the zombie apocalypse in Z Nation. For example, they might prevent a key scientific experiment from going wrong that initially started the spread of the virus. Another could be a character from the future coming back to the Z Nation timeline to retrieve an important artifact that is crucial for the survival of humanity in their own time. And a third plot twist might be a time - traveling character realizing that they are actually the cause of the future problems in Z Nation by their actions in the past.
One idea could be a crossover where Naruto characters end up in the Z Nation world. For example, Naruto with his shadow clone jutsu could be really useful against the zombies. He could send out clones to distract the zombies while the group tries to find a safe place.
In a fanfiction, 10K might get separated from his group. He then has to survive on his own in a very hostile environment filled with not only zombies but also other dangerous humans. During this time, he could learn new survival skills and also face some of his inner demons. He might even meet some new allies along the way who help him reunite with his group.
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 plotline could be that Addy somehow gets transported to the Naruto world. Maybe she has to learn about chakra and ninjutsu while also using her skills from Z Nation to survive in this new and strange place.
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.