Perhaps an old enemy of Naruto who has escaped from prison or some hidden place. They might steal his wife to lure Naruto into a trap so they can finally defeat him. Another possibility is a jealous suitor who couldn't stand seeing Naruto happy with his wife and decided to take her away.
It could be a powerful new villain who wants to get back at Naruto. Maybe someone from a rival village who thinks stealing his wife will weaken him.
Well, in such a fanfic, it could lead to Naruto being heartbroken. He might go on a journey to find out why his wife was stolen and try to get her back. Maybe he discovers some sort of conspiracy or misunderstanding that led to this situation.
In some fanfictions, it could be Hinata. They have a strong connection in the original series, and it would be a natural progression for them to get married and start a family.
One possible reason could be an enemy attack. A powerful and malicious foe might target Naruto's family to get back at him, and his wife could unfortunately become a casualty in the battle.
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.
Another possibility could be a new character. For example, a foreign ninja who has different skills and a unique personality. She could meet Naruto during his travels, and they could share a deep connection based on mutual respect and admiration for each other's abilities. This could lead to them falling in love and getting married.
Well, there are fanfics in this regard. However, the quality can vary greatly. Some might be beautifully written, delving deep into the characters' emotions and how they would interact as a couple. For example, they could explore how Naruto's positive attitude might influence Haku who has had a rather tough life. You really need to explore and read a few to find the ones that suit your taste.
In such a fanfic, Naruto might initially be in a state of deep shock and denial. He could isolate himself from his friends and the village for a while, brooding over the loss and constantly replaying memories of his wife in his mind.
Perhaps they would go on adventures together. They could explore new lands outside of the known ninja villages, facing various challenges along the way. Haku's skills as a ninja would complement Naruto's, and they'd make a great team.
One common theme is the exploration of their relationship against the backdrop of the Naruto universe. Another could be how they overcome the challenges of Haku's past.
Another great fanfic in this category is 'A New Chapter for Naruto and His Love'. This fanfic not only shows their relationship but also explores how their union affects the political and social aspects of the Hidden Leaf Village. It has an interesting plot that combines love, family, and village politics.