Another possibility is that there was some sort of magic or spell involved that made it seem like Ginny was pregnant, but it wasn't a normal conception. In that case, there might not be a traditional 'father' in the sense we think. It could be a plot device by a dark wizard to cause chaos in Harry and Ginny's relationship, and the baby might be part of some elaborate magical scheme.
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.
It could be someone completely new, an original character created for the fanfiction. Maybe a mysterious wizard who had a brief encounter with Ginny.
It could also lead to a mystery aspect in the fanfiction. Harry, being the hero he is, might start to investigate who the real father is. This could uncover a lot of secrets in the wizarding world, secrets that might be related to old families, forgotten spells, or hidden alliances. The whole story could then spiral into a complex web of mystery and drama as Harry delves deeper into the truth.
It could be one of Harry's enemies who is trying to frame him. So they set up the situation to make it seem like Harry is the father.
One plot twist could be that Harry is actually protecting Ginny and the real father. He might know some dark secret about the father that would put them in danger if it were known, so he pretends to be in the dark about the situation.
It could be Ron. Maybe they had a falling out or some complicated situation that led to Hermione keeping the baby a secret from him.
Another option is that the father could be Harry, but in a very different and alternative take on the story. Maybe there was a situation where they got closer than friends during a difficult time and it led to the baby. However, this would be a very radical departure from the original relationships in the series.
It could be a pure - blood wizard from an old family in the Slytherin circle. Maybe someone like Theodore Nott. They could have been in a relationship that was kept quiet due to family politics or personal reasons.
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.
Another option could be an alien from Starfire's home planet. This would add an interesting dynamic as the baby would have a strong connection to Starfire's alien heritage. It could also bring in some unique storylines about the alien culture and how it affects the baby's upbringing. Maybe the alien father has certain powers or responsibilities that are passed down to the baby.
Maybe it's a completely new character, someone from a different part of the wizarding world. He could be a mysterious wizard with a special talent or power that Rose was drawn to. This would open up a whole new set of plotlines as the reader discovers more about this unknown character and his connection to Rose.