Maybe it could be a new character introduced specifically for the fanfiction. This new character could bring a fresh dynamic to the story and create unique situations regarding Ino's pregnancy.
It could be Sai. Since they work together and have developed a relationship over time, it's possible that they could end up having a child together.
One possible storyline could be that Naruto and Ino start off as friends who gradually realize their deeper feelings for each other. Maybe they go on a mission together and during that time, they face a dangerous situation that makes them see how much they mean to one another. After getting married, they discover they are expecting a baby. Ino would be a very protective mother, using her mind - related jutsu to keep the baby safe even before it's born. Naruto, on the other hand, would be super excited and probably start training early on how to teach the baby ninjutsu.
Well, one possibility could be that Naruto and Ino accidentally end up having a baby in the fanfiction world. Maybe it's due to some kind of magical or unforeseen circumstance. Then the story could focus on how they deal with the new addition to their lives, like Naruto trying to balance his ninja duties with being a father and Ino using her medical and mental skills to take care of the baby.
Perhaps in this fanfiction, there's some sort of magical or chakra - related accident. A strange jutsu goes wrong and creates a baby that has a connection to both Ino and Naruto. They are then forced to figure out how to deal with this new life. Ino, with her knowledge of medical ninjutsu, helps to take care of the baby's health, and Naruto provides protection. Along the way, they form a family - like bond and the story could explore their relationship development.
Another possibility is Clark Kent himself. Their friendship was very strong, and there were always some unspoken feelings between them. A situation where Clark is the father would bring about a lot of emotional turmoil as they both try to figure out how to handle this unexpected development.
One possible plot could be that Naruto and Ino's baby has unique powers. Since Naruto has the Nine - Tails' chakra and Ino is from the Yamanaka clan with mind - related abilities. Their baby might inherit a combination of these, like being able to communicate with both people and animals using a form of mind - link. The story could then focus on how they raise the baby in a world that might be a bit scared or intrigued by this new power.
It could be someone Ginny had a past relationship with before Harry. Maybe an old schoolmate or a character from a side plot in the fanfiction.
The baby itself could be the most interesting character. Since it's a combination of Naruto and Ino, it might have a very distinct personality. It could be curious like Naruto and have a strong will, yet also be intelligent and perceptive like Ino.
It could be a completely new character. Maybe Naruto met her during his travels, and for some reason, she had to entrust the baby to Naruto. This could be due to her being in danger or having some kind of mission that made it impossible for her to take care of the baby herself.
It could be Klaus. Given their complex relationship in the series, it's possible that in fanfiction, they had a moment that led to her pregnancy. Klaus has always had a certain fascination with Caroline, and this could be an interesting twist to their story.
It could be Orochimaru. He might be interested in Ino's unique abilities, like her mind - control jutsu, for his own evil experiments. So he kidnaps her, and Naruto has to fight against Orochimaru's snake - like techniques to save Ino.