Well, it could be about how Ben and Gwen adjust to their new roles as parents. Maybe they face challenges like protecting the baby from villains who might want to use the baby's special powers (assuming in the fanfic world the baby has some).
In the fanfiction, it might be that the baby is a key to some sort of prophecy. Ben and Gwen then have to not only raise the baby but also protect it from those who want to use it for their own gain. They could travel to different places to keep the baby safe and find out more about the prophecy.
Perhaps the story focuses on the baby growing up with unique abilities inherited from Ben and Gwen. It could show how they teach the baby to control those powers. They might also deal with normal parenting stuff like the baby's first steps, first words, all while still being the heroes we know.
Well, one possibility could be that the baby inherits some of Ben's alien powers in a really unique way. Maybe it's not as straightforward as having the Omnitrix, but has some latent abilities that start to show as it grows. And Gwen could be teaching the baby how to control any magical tendencies it might have, since she has her own magic skills.
Maybe there are some secret organizations that want to study the baby because it's a hybrid of Ben and Gwen's powers. These organizations might not have evil intentions at first, but their methods of trying to get the baby could make them antagonists as they go against Ben and Gwen's wishes to protect their child.
The baby could have Ben's adventurous spirit. Since Ben is always eager to jump into action and explore new things, the baby might show signs of that same enthusiasm from a very young age. Maybe it would constantly be trying to reach for Ben's Omnitrix - like a shiny object that fascinates it.
One possible plot could be that Arthur and Gwen face challenges in raising their baby in a world filled with political intrigue. Maybe there are those who see the baby as a threat or a tool for power. For example, some nobles might try to use the baby's influence to gain more land or favors. Arthur and Gwen would then have to protect their child while also ruling Camelot fairly.
One possible plot could be that Mal and Ben face challenges in raising their baby in a world where their kingdoms have different traditions. For example, Auradon might have very formal education and upbringing methods, while the Isle of the Lost has a more rough - and - tumble way. They have to find a middle ground to ensure their baby grows up well - adjusted.
The plot could center around the OC having a unique item that attracts the attention of both Ben and Gwen. This item might be a key to an ancient power or a device that can manipulate time. As they all try to figure out what the item is and how to use it, they face challenges from other alien races who also want the item. Ben uses his various alien forms to battle these foes, Gwen uses her intelligence and magic to solve the mysteries related to the item, and the OC provides insights based on its own knowledge of the item's origin.
Well, the plot could involve them adjusting to parenthood. Maybe they face challenges like protecting the baby from their enemies. Since they come from different backgrounds, they might also struggle with how to raise the baby with their combined cultures.
One possible plot could be that a new powerful alien threat emerges that neither Ben, Gwen nor Kevin can defeat alone. They have to combine their unique powers and skills. Ben could use his alien forms, Gwen her magic, and Kevin his Osmosian abilities. They start off not getting along well in the team - up, with some bickering about the best way to approach the enemy. But as the situation gets more dire, they learn to trust each other and work in harmony to defeat the foe.
In a fanfiction, it could be that an evil sorcerer casts a spell on Gwen and Arthur's unborn baby. The baby starts to show strange and dangerous symptoms. Merlin is the only one who can sense what's really wrong. He has to race against time to break the spell. He delves deep into his magic knowledge and also consults ancient magical texts. Along the way, he has to hide his actions from those in Camelot who still distrust magic, all while trying to save the baby and ensure the future of Camelot's royal family.
One possible plot could be that Older Ben, having experienced more of the Omnitrix's power and the perils of the universe, takes Younger Gwen under his wing to train her in her magical abilities. They might go on adventures together where Ben uses his alien forms and Gwen uses her magic to fight enemies. Along the way, they develop a deeper bond of friendship and mutual respect.