It could be Ginny. In the original series, they had a relationship. An unexpected turn of events could lead to her being the mother in this fanfiction.
One potential character could be Seamus Finnigan. He has a bit of a fiery personality, and it would be interesting to see how he would handle being a teenage father. Maybe he accidentally gets into a situation where he has to take on this role.
Some fanfictions might choose Hermione as the mother. This could create an interesting dynamic, as Hermione is Harry's best friend. It would bring a whole new set of relationship developments and challenges for Harry as a father.
In some fanfictions, it could be Sirius Black. Some fans like to imagine that Sirius had a deeper connection to James Potter's family than just being best friends. They might create a scenario where there was some sort of magic or prophecy involved that made Sirius more than just Harry's godfather, but his actual father in this alternate story line.
In this fanfiction, the 'Father of Monsters' might be an ancient wizard who has been in a deep slumber for centuries. He was awakened by some disturbance in the wizarding world and, feeling angry about the changes that have occurred, decides to create monsters to bring back what he believes was the 'true' order of the wizarding world. His knowledge of old magic gives him the ability to create extremely powerful and dangerous monsters that no one has ever seen before.
Another possibility could be Draco Malfoy. Think about it. In some fanfictions, they might rewrite the relationships completely. Draco has his own complex character arc, and perhaps in this 'harry potter is not the father' scenario, he ends up being the father due to some unforeseen circumstances, like a misfired spell that alters the course of events related to parentage.
It could be Sirius Black. Maybe there was some misinformation that made Harry believe Sirius was his father for a while, but then it turns out he isn't. After all, Sirius was very close to Harry and had a father - like relationship with him in some ways.
Maybe Ron Weasley. Imagine a fanfiction where Ron, after getting married, has triplets. Given his humorous and sometimes clumsy nature, it would be really fun to see how he and his wife deal with three little ones. He could pass on his love for food and his loyalty to his children in a really endearing way.
One possible plotline could be Harry struggling to balance his new role as a father with his studies at Hogwarts. He might face challenges from the school authorities who are not used to a teenage student being a father.
In the 'Harry Potter teenage father fanfiction', it could be really fascinating to see how the teenage father's family background plays a role. If he's from a pure - blood family, there might be different expectations compared to a half - blood or muggle - born. He might have to face the pressure of carrying on the family name and traditions while also taking care of his child. Also, how his relationship with his child's mother evolves over time is an interesting aspect to explore, whether they stay together or have to co - parent from a distance.
It could be Lily Potter. Since she sacrificed herself for Harry, her gift could be something like the love that still protects him, which could be explored in a more symbolic way in the fanfiction.