A possible plot is that Severus discovers a new potion that can enhance the magical abilities of the unborn child. Minerva is hesitant at first, but Severus convinces her to let him try it on a small scale. Meanwhile, they have to navigate the challenges of Minerva still teaching her classes and Severus doing his potions work. They also face some opposition from certain members of the wizarding community who think their relationship is inappropriate. But they stand strong together and look forward to the arrival of their baby.
Severus, who was always a bit brooding and reserved, might become more protective. His focus would shift from just his potions and the dark arts to ensuring Minerva's and the baby's safety. He would be constantly on guard. Minerva, on the other hand, usually a strict and no - nonsense teacher, could become more emotional. Small things might make her cry or get overly excited. She might also be a bit more lenient with her students as she experiences the softer side of life through her pregnancy.
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.
Another trope is the 'mentor - mentee' dynamic in reverse. Usually, they are both mentors to students, but in some fanfictions, they end up learning from each other. Snape might learn more about emotional openness from McGonagall, while she could learn about some of the darker arts and how to better protect students from Snape. It's an interesting flip of their usual roles.
For Minerva, you could explore her more emotional side. In fanfiction, show her struggling with her feelings for Severus. Maybe she has always seen him as a difficult colleague, but then realizes there is more to him. For Severus, delve deeper into his loyalty. You can write about how his loyalty to Dumbledore and Hogwarts extends to his relationship with Minerva. For example, he would do anything to protect her when Hogwarts is in danger because she is an important part of what he is protecting.
One idea could be a story where Minerva and Severus have to team up to protect Hogwarts during a new, unexpected threat. Maybe a dark magic artifact is discovered near the school and they must use their combined knowledge and skills to keep it from falling into the wrong hands.
One popular theme is their friendship. Despite their differences, they are both dedicated teachers at Hogwarts, and fanfictions often explore how they might support each other behind the scenes. Another theme is their possible romantic relationships, not necessarily with each other, but how their views on love and relationships might be. For example, Snape's unrequited love for Lily could be contrasted with McGonagall's more reserved approach to love. Also, their roles during the wizarding wars are often explored in fanfictions. How they coped with the danger, made decisions, and protected the students.
You can try fanfiction websites like Archive of Our Own (AO3). It has a large collection of fanfictions on various topics, including those related to Albus and Minerva. Just use the search bar and type in relevant keywords like 'Albus and Minerva pregnant' and you should get a list of available stories.
One great fanfiction could be 'The Miracle of Albus and Minerva'. It might explore how their relationship changes with the news of pregnancy, delving into their emotions and the reactions of the wizarding community. Another could be 'A New Chapter for Albus and Minerva' which focuses on their preparations for the new arrival while also dealing with their duties at Hogwarts.
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.