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.
There could also be a theme of self - discovery. As they interact, both rook and ben might discover new aspects of themselves. For example, rook might learn more about his own courage through his association with ben, and ben might understand the importance of loyalty better because of rook. This self - discovery can be a really interesting theme to explore in fanfic.
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.
You can try looking on some popular fanfic websites like Archive of Our Own (AO3). It has a wide range of fan - created content, and there's a good chance you might find 'ben x rook lemon fanfic' there if it exists.
One possible 'ben x rook lemon fanfic' could be about their adventures together in a post - apocalyptic world where they have to rely on each other. Their relationship deepens as they face challenges, and the lemon aspect could be a passionate moment they share during a quiet interlude in their journey.
I'm not sure about specific rook x ben fanfic titles off the top of my head. But you could also check out some fanfic communities on social media platforms. People might share their favorite rook x ben fanfics there or even give you some tips on where to find them.
Perhaps a fanfic where Ben and Rook accidentally swap abilities. Ben has to deal with Rook's weaponry and combat skills, while Rook tries to control the Omnitrix. It would be full of comical moments as they struggle to adapt, but also a chance for them to learn more about each other's strengths and weaknesses.
Another way is to follow some fanfic writers who are known for writing about these characters. They may post new 'ben x rook fanfic' on their personal blogs or on writing platforms. You can discover them through social media or by searching for Ben 10 fanfic writers.
Another theme is adventure. Since they are from a universe full of strange planets and creatures, many fanfics take them on exciting adventures across the galaxy. They might be searching for a powerful artifact or trying to stop an evil force from taking over the universe. It's all about the journey and the discovery along the way.