One idea could be a story where the Thundermans have to team up with their arch - enemies to face a new, more powerful threat. Maybe a super - villain from another dimension arrives and their combined powers are the only way to stop it.
In 'Thundermans' pregnant fanfics, a frequent theme is the adjustment of the family dynamics. The arrival of a new baby, especially in a super - powered family, can change the relationships between family members. There could be jealousy among siblings, or a new - found sense of responsibility. Another theme could be the exploration of the pregnant character's fears and insecurities. For example, they might be worried about how their superpowers will affect the pregnancy or how they will be able to take care of the baby while still being a superhero. And of course, the theme of love and protection within the family is also quite common, as the family rallies around the pregnant character.
First, you need to decide on a basic plot. It could be something simple like a new adventure for the Thundermans. Then, think about the characters you want to focus on. Are you more interested in Max or Phoebe? Once you have that sorted, just start writing. You can begin with a description of the setting, like 'The Thundermans' house was bustling with activity as they prepared for another day of superhero - and - villain - chasing.'
Most likely, it isn't. Usually, such characters or stories are crafted from the imagination of the author rather than being rooted in reality. There might be some elements inspired by real-life, but it's not a direct retelling of a true story.