It could be someone from the other universe in the crossover. For example, if it's a crossover with a superhero universe, it could be a character like Superman. The combination of Naruto's strong ninja genes and Superman's superpowers would make for an extremely powerful child. Or it could be a character from a high - fantasy world, like a powerful mage. Their union would bring about a child with a mix of magic and ninja abilities.
It could be Sasuke. Their relationship has always been complex in the original Naruto story. In the fanfiction, their relationship might have developed into a romantic one, leading to the birth of the child. The child would then inherit traits from both of them, perhaps Sasuke's cool-headedness and Fem Naruto's determination.
One possible plot could be that in this crossover, Female Naruto is in a world where she has to protect her child from an unknown enemy. Maybe it's a new villain from another universe. She would use her ninja skills, which are already quite powerful, but with the added motivation of protecting her child, she becomes even more fierce. Her child could also inherit some unique powers, either from Naruto or from the other universe in the crossover, creating an interesting dynamic as they both try to survive and thrive in this new situation.
It could be a mortal man that Artemis fell in love with in secret. Maybe a skilled hunter who caught her eye in the forest one day.
The child could be an orphan from a small, destroyed village that Naruto comes across during one of his missions. This child might have lost their family to a powerful enemy and Naruto feels a sense of responsibility to take them in.
Perhaps it could be a new character. A mysterious wizard who comes into Hermione's life, and they fall in love and have a child. This would add an element of newness and mystery to the fanfiction, as the readers would get to learn about this new character and his relationship with Hermione.
The villagers themselves could be responsible. Their constant ostracization and negative treatment of Naruto could lead to his downfall. He might be pushed to a point where he takes risks that ultimately cost him his life.
In a crossover with the Attack on Titan world, characters like Eren could interact with Naruto. Eren, with his strong will to fight for freedom, could learn from Naruto's experience as a father and a hero. Levi might also be interested in Naruto's combat skills and how he trains his child in the ways of the ninja.
Another option could be a new character. A mysterious and powerful ninja from another village who Fem Naruto met during a diplomatic mission. They had a short - lived romance, and he doesn't even know about the child. This would add an element of mystery and the potential for the father to show up later in the story.
It could be a heroic character she meets during her quest. Maybe a powerful wizard or a brave knight who has a brief but intense encounter with Lucy, leading to her pregnancy.
In fanfiction, the possibilities for Naruto's father could be many. It could be an original character created by the author. Maybe a powerful shinobi who has a connection with the female Shinigami. For example, he could be a shinobi who made a deal with the Shinigami in the past and through some convoluted plot, became Naruto's father.