The fanfiction could start with the Fire Daimyo proposing the marriage as a way to bring more stability to his rule. Naruto, being the selfless person he is, agrees for the greater good. Their relationship begins on a rather formal basis. But as they start spending time together, they discover each other's hidden qualities. The daughter might be interested in Naruto's tales of heroism and his unique ninja skills. Naruto, on the other hand, might be charmed by her intelligence and grace. This could lead to a deep and meaningful relationship, with them facing various challenges together, like threats from other lands or internal political unrest in the Land of Fire.
Well, in such a fanfiction, perhaps Naruto is pressured into the marriage for the sake of peace in the Land of Fire. The Fire Daimyo's daughter could be a strong - willed character. At first, Naruto and she might clash due to their different upbringings. However, as they get to know each other better, they find common ground. Naruto could teach her about the ninja way, and she could introduce him to the more refined aspects of the daimyo's court life, leading to a loving relationship.