The immediate effect would be a huge shock to Naruto. His trust in Tsunade would be shattered, and this could lead to a period of self - doubt for him. He might question his own judgment in trusting people.
In the long run, it could change Naruto's entire outlook on life and his goals. He might become more cynical and less trusting. His journey to become Hokage could also be affected as he now has to deal with this betrayal and its consequences. He may have to prove himself even more not only to the village but also to himself that he can overcome this setback and still be a great leader.
The prank could also lead to an unexpected alliance. Maybe after the prank, Android 18 realizes that Naruto is just a fun - loving guy and they end up teaming up for some other adventure. They could combine their powers in unique ways. For example, Android 18's strength and Naruto's ninjutsu could be used to take on a powerful enemy. This would completely shift the direction of the fanfic's storyline.
The storyline would likely start with Naruto's disillusionment with Konoha. His actions would then shock his friends. Sasuke, for example, might initially be confused but then choose a side. There would be intense battles within the village as Naruto tries to fight against the forces that he now opposes. His former friends and comrades would be torn between loyalty to the village and their friendship with Naruto.
It could also have an impact on the overall plot. For example, other characters' reactions to Naruto being a girl could be explored. Some might be accepting, while others might be shocked or even hostile. This could create new conflicts and alliances within the fanfic. Additionally, Naruto's own journey might change as she now has to deal with Sasuke knowing her secret and how that affects her standing in the ninja world.
One possible reason could be power. Maybe in the fanfic, there's a new source of power that Tsunade wants desperately and Naruto stands in her way to obtain it. So she betrays him to get to that power.
At first, Naruto would likely be extremely shocked. He has always considered Sakura a friend and teammate, so this would be a huge blow. He might question his own judgment in trusting her.
However, Naruto also has a big heart. In the long run, he would probably try to find a way to save Kakashi or bring him back to the right side. He wouldn't give up on Kakashi easily, believing that there must be some underlying reason for his actions and that he can be redeemed.
Their relationship might also cause some friction in the village politics. Some traditionalist shinobi might not like the fact that Naruto, a young and sometimes brash ninja, is on equal footing with Tsunade, a respected veteran. This could lead to power struggles within the village, with different factions supporting either Naruto or Tsunade. However, if they manage to work together despite the differences, they could bring about a more balanced and inclusive political environment in the Hidden Leaf.
Naruto would likely be initially shocked. He has always considered the Kyubii as a part of him and a friend in some ways. So the betrayal would be a huge blow to his sense of trust.
If Tsunade hurts Naruto, it could cause Naruto to become more distant from her. He might avoid her, and this could have an impact on the village as well. Tsunade, on her part, would probably feel a great deal of guilt. This guilt could make her try to make amends, but Naruto may not be so quick to forgive. Their relationship would be in a state of flux, with Tsunade trying to regain Naruto's trust and Naruto trying to come to terms with what happened.
The storyline would be greatly affected. In terms of character development, Naruto would have to learn a lot about Sakura, her insecurities, and her strengths. This new understanding could then be carried over when he turns back to his normal self. In the plot, it could also lead to new alliances or enemies. For example, if Naruto - as - Sakura accidentally offends someone who Sakura had a delicate relationship with, it could create new conflicts. Or, on the other hand, he could make new friends by showing a different side of Sakura that others hadn't seen before.