Perhaps an old enemy from Naruto's past that was thought to be defeated has resurfaced. This enemy might have targeted Naruto's wife as a way to get revenge on Naruto for their previous defeats. They could be using her as a bait to lure Naruto into a trap.
One possible storyline could be that Naruto's wife was killed during a major battle against a new and powerful enemy. This would send Naruto into a deep state of grief and depression, perhaps causing him to question his purpose as a shinobi. He might then embark on a journey to find the true cause of her death and seek revenge.
His relationship with Sasuke could become very different. Without the memories of their rivalry and friendship, Sasuke might see Naruto as a completely different person, and Naruto might not understand Sasuke's complex feelings towards him at all.
Well, in such a fanfiction, Naruto might find himself in a strange new land where he has no idea how to get back home. Maybe he loses his memories and has to start from scratch to figure out who he is and what his purpose is.
One aspect to consider in this Naruto and Haku fanfiction where she's his wife is the cultural implications within the Naruto universe. In the ninja world, marriage is not just a union of two people but also has implications for clans and villages. Naruto being a prominent figure in Konoha and Haku having her own background (even though she was initially from the Land of Water). Their marriage could be a symbol of unity between different regions or even a way to bridge the gap between different ninja ideologies.
Sasuke could be one of them. Despite their past differences, Sasuke might sense that Naruto is in trouble and come to his aid. After all, they have a deep connection.
Hinata could be a likely candidate in such a fanfiction. Since she is Naruto's real - life wife in the original series. Writers might twist their relationship in fanfiction and create a scenario where she betrays him. Maybe due to some family pressure or a misunderstanding that gets out of hand.
An alien race that is highly intelligent but physically weak. They see Naruto as a powerful protector. In return for Naruto's protection, they offer him their advanced technology which could be crucial in his journey back to his world. For example, they might give him a device that can create wormholes to different parts of the universe.
One interesting plot could be Naruto being found by a hidden group who wants to use his power for their own ends while he's in his memory - lost state. They could manipulate him into believing he's one of them.
His relationships with his friends like Sasuke and Sakura would be deeply affected. They might feel like they've lost a part of Naruto. Sasuke, for instance, might initially be frustrated as he's used to Naruto's determination. Now, with his memories gone, Naruto might seem like a different person to Sasuke.