It could be one of Naruto's former teammates from a mission. Maybe this teammate has some hidden motives or has been bribed by an enemy. They could spread false information about Naruto within the community, making it seem as if Naruto has betrayed them all during a crucial mission. This would cause the rest of the community to lose trust in Naruto.
There's a chance that some of his former teammates who have their own insecurities. For example, someone who always felt in Naruto's shadow might be the one to betray him in order to get more recognition for themselves.
He could betray Naruto to Orochimaru. Orochimaru is always looking for powerful allies and new test subjects. Lucifuge might be lured by Orochimaru's promises of power and knowledge. Orochimaru could use Lucifuge to get closer to Naruto in order to steal the Nine - Tails' chakra or some other valuable asset.
It could be Zabuza. He might pretend to be on Naruto's side at first during the Wave events and then betray him when Naruto least expects it. Zabuza is a mercenary and his loyalty can be easily swayed.
Even Hinata could potentially be written as a betrayer in some really out - there fanfictions. Maybe in a crossover where there are mind - control elements, Hinata could be forced to turn against Naruto. But this would be a very different take on her character as she's usually portrayed as extremely loyal to Naruto.
It could be Sasuke. He might be jealous of Naruto's new power and feel overshadowed. So at Wave, he decides to betray Naruto to prove his own worth.
Sasuke could be a likely candidate. He's always been somewhat jealous of Naruto's growth and popularity. In the stress of the Forest of Death, he might let that jealousy get the better of him and betray Naruto. Maybe he thinks that by getting rid of Naruto, he can be the only strong one in the team.
It could also be related to some hidden family secrets or prophecies. Maybe there was a prophecy that Naruto would bring great destruction in the future and Kushina, believing in it, thought she had to stop him before it was too late. This would be a very complex and heart - wrenching betrayal if it was based on such a false premonition.
The village elders are likely candidates. They are often shown to be power - hungry and manipulative in the Naruto series. They might see Naruto's power as a threat to their control over Konoha and thus plot against him, which could lead to Naruto feeling betrayed.
In some really complex fanfic plots, there could be a prophecy or some sort of ancient power play involved. Suppose there is a prophecy that Naruto will bring great destruction in the future, and Kushina is made aware of it through some dark magic or secret organization. She might think that betraying Naruto now is the only way to save the village and the world, even though it goes against her motherly instincts. This would be a very convoluted and interesting take on the idea of her betrayal.
It could be his fellow team members. They might betray him out of greed for power. Maybe they thought that by getting rid of Issei, they could gain more resources or a higher position within their group.