One idea could be that the girlfriend, perhaps under the influence of an enemy, reveals Naruto's bloodline secrets to those who would use it against him. For example, she might be tricked into thinking she's helping Naruto in some way, but actually endangering him.
In a more complex scenario, the girlfriend could be from a clan with a long - standing feud against Naruto's bloodline. Despite her feelings for Naruto, her family's pressure and loyalty to her clan force her to betray Naruto's bloodline secret. This would create a lot of internal conflict for her character as she struggles between love and family obligation.
He might be initially shocked and heartbroken. Naruto has always been someone who values loyalty and friendship deeply, so this betrayal from his girlfriend would be a huge blow. He could go into a state of self - doubt, wondering if there was something he did wrong to make her do such a thing.
One possible scenario could be that Naruto's girlfriend betrays him because she is jealous of his godlike powers. Maybe she feels overshadowed and in a moment of weakness, she aligns herself with Naruto's enemies. It could lead to a great internal conflict for Naruto, having to face not only external threats but also the pain of betrayal from someone he loved.
In a more complex scenario, Naruto could be influenced by an outside force. Let's say an enemy of Konoha uses mind - control or some form of manipulation on Naruto. They make him believe that the ANBU is the enemy of the village and that he needs to take them down. As he betrays the ANBU, he slowly starts to realize that he's been used, but by then he's already in too deep and has to find a way to fix the situation and regain the trust of his friends and the village.
One idea could be a character in the Naruto universe who mysteriously obtains the Geass bloodline. This could lead to a whole new set of powers and conflicts. For example, they might use the Geass power to control powerful ninja beasts or even other ninjas in the heat of battle, creating a moral dilemma as Geass often comes with a price.
He could then feel a deep sense of anger. Naruto has a temper, especially when it comes to those he loves. He might vow to find out the reason for the betrayal and make things right, or if it was truly malicious, seek revenge in his own Naruto - like way, which would involve trying to understand the person's motives even while being angry.
One idea could be that their own village betrays them. Maybe there's a power - hungry leader who sees Naruto and his sister as threats to their new regime. They could be framed for a crime they didn't commit and then forced to flee the village. In the process, they might have to rely on some unlikely allies they meet on their journey, like a group of outcasts who also have a grudge against the village.
One idea could be that in the fanfiction, Hinata is under the influence of a powerful genjutsu. This genjutsu makes her see Naruto as a threat to the Hidden Leaf village. So, she betrays him to protect the village in her distorted view. It could lead to a lot of inner conflict for Hinata once she realizes the truth and Naruto having to deal with the pain of her betrayal.
The story could explore how Naruto's power is affected by this betrayal. Maybe his godlike powers start to waver because his emotional state is so unstable. He might have to go on a journey to regain his confidence and control over his powers. Along the way, he could meet new characters who help him understand that not all relationships are doomed to end in betrayal. This would be a great way to develop Naruto's character further in the fanfiction.
One possible reason could be that she was forced to betray him. Maybe an enemy got to her family and threatened her to turn on Naruto. For example, in the Naruto universe, there are many powerful villains who would stoop to such tactics.
One idea could be a story where a new character with the lava release bloodline enters the Hidden Leaf Village. This character could initially be seen as a threat but later prove to be an ally. They might have unique abilities within the lava release, like creating lava constructs that can be used for both offense and defense.