One possible fanfiction could be about Ben and Kai going on a new adventure in the Omniverse. Maybe they discover a new alien species that is causing chaos in a far - flung galaxy. Ben uses his Omnitrix to transform into different aliens to try and stop the threat, while Kai uses her martial arts skills to fight alongside him.
Well, in a fanfiction, they could be on a mission to save a planet that is on the verge of destruction. Ben uses his alien powers to hold off the immediate threats, like asteroids or attacking creatures, while Kai tries to find the root cause of the planet's problems, which could be some sort of evil technology or a corrupt leader.
One great Ben x Kevin fanfiction could be about their adventures in a parallel dimension. In this story, they might face new aliens and challenges that test their friendship even more. Maybe they find a powerful alien artifact that only works when they combine their unique abilities, leading to a deeper understanding of each other.
Well, if Ben was kidnapped in fanfiction, his friends would probably start a huge rescue mission. Gwen would use her magic and Kevin his powers to track him down. They'd face all sorts of challenges along the way, like bad guys trying to stop them or traps set by the kidnappers.
One possible plot could be that Rook and Ben go on an adventure to a new planet in the Ben 10 Omniverse. They encounter a strange alien species that has a unique power. Ben tries to use his Omnitrix to transform into an appropriate alien to deal with the situation, but there are some glitches. Rook, with his Plumber skills, helps Ben figure out what's wrong and together they overcome the challenges and make new friends among the aliens.
There's also a chance that it could be a former enemy of Ben who has a grudge. They might want to get revenge on Ben by kidnapping him and putting him in a difficult situation. It could also be a mad scientist who wants to study Ben's alien forms and the Omnitrix, so they kidnap him to do all sorts of experiments on him.
The main characters are obviously Ben and Kai. Ben is the wielder of the Omnitrix, which gives him the power to transform into various alien forms. Kai is a strong - willed and adventurous character who often gets involved in Ben's escapades.
Ben is an obvious choice as he is the main character. His different alien forms offer a lot of possibilities for storylines. Gwen is also great as she has her own powers and a complex relationship with Ben. And of course, Kevin, with his unique abilities and his friendship - turned - rivalry - turned - friendship again with Ben.
Well, you could write a 'ben 10 omniverse fanfiction' where Ben loses his Omnitrix and has to rely on his wits and the help of his friends to still fight the bad guys. Or perhaps a story where the Omnitrix starts evolving on its own and gives Ben new and unexpected powers. Also, a plot where an old enemy from the past returns with a new and more dangerous plan.
Another way is by showing their daily interactions. Like when they're just hanging out between missions, sharing stories about their different worlds. Ben could tell Rook about his adventures on Earth, and Rook could share stories about his Plumber training. This would make them understand each other better and grow closer. They could also have inside jokes that develop over time from these interactions. Also, having them train together to improve their combat skills would not only make them better fighters but also strengthen their relationship as they rely on each other during the training sessions.