There are many possibilities. It could be a mysterious and powerful ninja that no one expected. Since it's fanfiction, the author could come up with an entirely new character who has unique abilities and a complex backstory, and that person could be Naruto's father. Another option could be an anti - hero type of character, someone who has a shady past but still has a connection with Rouge that led to Naruto's birth.
Well, it could be anyone really. It might be some random ninja that Rouge had a relationship with. Or it could be a character created specifically for this fanfiction. Maybe it could even be a character from another village who had a secret liaison with Rouge.