Another option is that the mother is a mysterious witch from a far - off land. Sirius might have met her during one of his travels when he was on the run from the Ministry. They had a short - lived relationship, and she left the baby with Sirius because she knew he was from a powerful wizarding family and could protect the child better in the wizarding world he knew.
One possible plot could be that Sirius Black discovers he has a baby unexpectedly. Maybe it was a result of a past relationship he had before his time in Azkaban. He then has to learn how to be a father while also dealing with the wizarding world's reaction to his new situation, as many might still be suspicious of him.
In some fanfictions, the mother could be a character that was only briefly mentioned in the original series. For example, it could be a witch he knew from his school days at Hogwarts. They reconnected later in life and had a daughter. Or, it could be a new character who has a special connection to the Black family, perhaps a distant relative who got closer to Sirius and they had a child together.
It could be a completely new character created for the fanfiction. Maybe a powerful witch who met Sirius during his time on the run. She could have been helping him and they fell in love, and then had a kid.
In this fanfiction, the mother could be a powerful sorceress. She used her magic to create a baby with Kratos' essence for some grand purpose. But then she disappeared, leaving Kratos to deal with the baby. This would add an interesting layer of mystery to the story as Kratos tries to figure out why she did it and what the baby's true nature and purpose are.
Perhaps it could be a character from an alternate dimension. In some fanfiction scenarios, portals open between different dimensions, and Spiderman could have had an encounter with a female character from another world, which led to the baby.
It could be someone from Stiles' past that he had a brief relationship with. Maybe a girl he met during a summer vacation or a girl from another town that he had a fling with and then lost contact with until she shows up with the baby.
Another option could be an original character created just for the fanfiction. This gives the writer a lot of freedom to create a character with unique traits and a backstory that fits the plot they have in mind for Draco and the baby.
It could be Hinata. Since they got married in the original series, it would be a natural choice for her to be the mother in the fanfiction.
It could be an original character created just for this fanfiction. Maybe a girl who Stiles met during a summer vacation and had a brief fling with.
It could be Mary Jane. Since they have a long - standing relationship in the Spider - Man universe, it's quite possible that in the fanfiction she could be the mother of the baby.