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.
One interesting storyline could be a female version of Ben having to deal with different alien cultures in the Omniverse in a more diplomatic way than Ben usually does. For example, she might use her charm and intelligence to resolve conflicts between warring alien species on a planet instead of just using brute force with the Omnitrix transformations.
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.
One popular character could be Gwen from the Ben 10 series, especially when she is incorporated into Omniverse fanfiction. She has her own set of powers and a strong personality. In fanfiction, she can be a great support or even a rival to the female Ben.
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.
I haven't actually read this specific fanfic, but generally in a 'Ben x Esther' fanfic in the Ben 10 Omniverse setting, it might explore their relationship in different ways. It could involve adventures where they team up, and their characters' growth while dealing with various threats in the Omniverse. Maybe it also delves into how their different personalities interact and complement each other.