It could be a fellow Hogwarts professor. Maybe Flitwick, as they've worked together for a long time and have a mutual respect for each other. Their relationship could have developed secretly over the years.
Yes, there are some out there. You can try searching on fanfiction platforms like Archive of Our Own. Some might focus on how she manages her pregnancy while still being a strict professor at Hogwarts, adding a touch of personal drama to her usually composed character.
One plot idea could be that she tries to keep her pregnancy a secret at first, fearing it might affect her position at Hogwarts. But as her condition becomes more obvious, she has to face the reactions of the students and staff.
She becomes more nurturing. Usually, she's known for her strictness at Hogwarts, but when she adopts Harry, she softens up. For instance, she might show more patience when Harry makes mistakes in his magic studies.
One plot idea could be how she manages her pregnancy while still fulfilling her duties at Hogwarts. Maybe she has to keep it a secret at first and enlists the help of a few trusted colleagues like Dumbledore. Another could be exploring her relationship with the father, whether it's a fellow wizard or a Muggle, and how that relationship evolves during the pregnancy.
There are probably fanfictions out there with this premise. Fanfic authors love to play with the established canon of a story. By making Minerva McGonagall Hermione's mother, they can rewrite many aspects of Hermione's life. Maybe she would have grown up with a different set of values, or her magical abilities could be seen in a new light. It also has the potential to create new conflicts and reconciliations within the Hogwarts community.
One great Severus Snape and Minerva McGonagall fanfiction is 'The Unlikely Alliance'. In this story, they are forced to work together on a Hogwarts project. Their different teaching styles and personalities create interesting interactions. Snape's brooding nature contrasts with McGonagall's more no - nonsense approach, and as they overcome obstacles in the project, they also start to understand each other better on a personal level.
One great 'Harry Potter Minerva McGonagall' fanfiction is 'The Transfiguration of McGonagall'. It delves into her early years as a teacher and how she dealt with the challenges. Another interesting one is 'McGonagall's Secret Mission' which shows her on a covert operation during the wizarding war. There's also 'Minerva's Heart' that focuses on her relationships with other Hogwarts staff members.
The world of Harry Potter itself is so rich and detailed. Minerva McGonagall is an integral part of that world. Fanfiction about her allows readers to return to the magical world of Hogwarts. Writers can also introduce new magical elements or storylines related to her. For example, they can create stories about her adventures during her youth, or her role in some new magical crisis that was not part of the original books. This gives fans a chance to experience more of the magic and the character they love.
You can start by looking on fanfiction platforms like Archive of Our Own (AO3). It has a vast collection of Minerva McGonagall fanfictions, with various tags to help you narrow down your search. Another option is Fanfiction.net, which also has a lot of works related to her.