Perhaps the baby's existence causes a rift in the wizarding world. Some might view it as an abomination due to the age and past relationship between Hermione and Severus. This could lead to Hermione and Severus having to fight not only for their baby's safety but also for their right to be a family in a world full of prejudice.
One possible plot twist could be that the baby has some unique magical abilities that neither Severus nor Hermione expected. Maybe the baby can communicate with magical creatures in a way no one has seen before. This could lead to all kinds of adventures as they try to understand and control this new ability.
Well, perhaps the phoenix is actually a guardian of a powerful magic that only Hermione and Severus can unlock together. But there's a catch. They have to overcome their initial distrust and dislike for each other. This would lead to a lot of tense and interesting scenes as they try to figure out how to work together without killing each other first.
A great plot twist could be that younger Snape actually has some knowledge of Hermione's time - traveling. Maybe he's been having strange dreams about the future and when he meets Hermione, he realizes she's part of it. This would completely change the power dynamic between them.
One possible plot twist could be that they face unexpected magical difficulties. For example, a spell gone wrong might temporarily affect their chances of having a baby. Maybe a dark magic remnant in their house, left over from their previous battles, starts to interfere with the natural process. Another twist could be that they discover a long - lost family curse that might be passed on to their potential child. This could lead them on a journey to find a cure before they actually conceive.
In the fanfiction, it could be that the baby is a result of some sort of magical accident. Hermione then has to deal with the challenges of raising a child she didn't plan for. She might have to search for a way to reverse the situation if she feels she's not ready, or she could embrace motherhood and learn to adapt her life around the baby's needs.
Maybe the story could be about Hermione trying to protect the baby from dark forces in the wizarding world while keeping its existence a secret. She might have to face challenges alone without the support of her friends at first.
In some fanfics, a plot twist could be that Severus was actually protecting Hermione all along under Dumbledore's secret orders, and Sirius discovers this and has to re - evaluate his hatred for Severus. This completely changes the dynamic between the three.
One possible plot could be Severus saving baby Draco from a dangerous situation. Maybe there's a dark wizard targeting the Malfoy family and Severus steps in to protect Draco. This could start a special connection between them.
One possible plot twist could be that at first, Hermione and Severus despise each other due to their past differences. For example, Severus has always been rather cold towards Gryffindors, and Hermione is a proud Gryffindor. But as they are forced to spend time together because of the arranged marriage, they start to discover each other's hidden depths. Hermione might find Severus's intelligence and his complex past with Lily fascinating. And Severus could come to respect Hermione's quick wit and thirst for knowledge. Another plot twist could be that there is some external force trying to break up the arranged marriage. Maybe some dark wizards see it as a threat to their plans, or there are those in the wizarding world who oppose the union of a Muggle - born like Hermione and a former Death Eater like Severus.
One idea could be that Severus and Hermione have to protect their baby from dark forces. Maybe there are some old enemies of Severus who target the baby to get revenge. Another plot could be the baby having some unique magical abilities that they need to figure out how to handle and train.