In the 'naruto heir harem fanfiction', a possible plot could be that the heir discovers a long - lost power. The members of the harem are initially skeptical or even a bit jealous. But as he learns to control this power, they come to support him. Along the way, there are cultural differences among the harem members, like different views on chakra usage or ninja traditions, which add to the story's complexity and the heir's journey to bring them all together.
One idea could be that the heir, as Naruto's descendant perhaps, has to navigate the complex relationships within the harem. Each member of the harem could represent a different clan or group, and he has to balance their interests while also dealing with his own growth as a ninja.
Maybe the heir in the fanfiction is faced with a threat to the village, and the members of his harem each have unique skills that can help. For example, one might be an expert in healing, another in stealth, and he has to lead them in a strategy to protect the village. It could also involve some sort of love triangle or multiple relationship dramas within the harem during this time of crisis.