A cool idea would be to have Rook and Ben get trapped in a time - loop within the Ben 10 Omniverse. Every day they wake up and the same events happen, but they start to notice small differences. They have to figure out how to break the loop. Along the way, they learn more about each other and themselves, and also discover some hidden truths about the Omniverse.
An interesting aspect could be exploring Rook's past more deeply. Maybe he has some secrets from his time before being a Plumber that are related to the Ben 10 Omniverse. Ben could help him uncover these secrets while they are on a mission. For example, they could be investigating an ancient alien artifact, and as they dig deeper, Rook's past starts to unfold.
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.
Perhaps a story where an old enemy of Ben's targets Rook to get to Ben. Ben feels guilty and protective. They go on the run, hiding in different places across the galaxy. Along the way, they meet some interesting characters who help them. In the end, they face the enemy together, with Ben using his Omnitrix and Rook using his upgraded Proto - Tool to defeat the bad guy.
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.
One possible plot could be that Esther gets caught up in an alien conspiracy in the Omniverse. Ben, being the hero, steps in to save her. As they work together to uncover the truth behind the conspiracy, they start to fall for each other.
Well, in a 'Ben 10 Omniverse Ben x Kai' fanfic, perhaps it could start with Kai joining Ben on one of his inter - dimensional adventures. Along the way, they encounter various challenges that test their relationship. Ben might use his Omnitrix in creative ways to protect Kai, and in the process, they realize their mutual attraction. For example, when they are trapped in a strange alien world, Ben has to transform into different aliens to find a way out while also keeping Kai safe from the local dangers.
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.
By having them face a common enemy. When they fight side by side against a powerful foe, they can learn to trust each other more and understand each other's fighting styles better.
In Ben 10 Omniverse fanfics, the relationship between Ben and Gwen can be really interesting. Usually, they have a very strong bond as cousins. In some fanfics, their relationship might be explored more deeply, like how they support each other during battles. Ben with his Omnitrix powers and Gwen with her magic often make a great team, and fanfics can show their interactions in new and exciting ways, such as when they face a new and extremely powerful enemy that they have to combine their skills to defeat.
Adventure is likely to be a theme as well. The Ben 10 Omniverse is full of exciting places and situations. Fanfics may center around Attwa and Ben Lemon going on adventures, using their unique abilities or powers to overcome obstacles.
A more character - driven plot could be about Ben trying to help an alien species that is being persecuted in the Omniverse. He uses his different alien forms to understand their culture and problems better. Along the way, he discovers that there are some dark secrets in the Omniverse's politics that are related to this species' situation. This would allow for a deeper exploration of the Omniverse's world - building and Ben's character growth.