Rook's character could develop by becoming more in touch with his emotions. Usually, he is very by - the - book as a Plumber. But being around Rayona, he might start to take more risks and think outside the box. For Rayona, she could become more confident in her abilities. Maybe she starts off a bit unsure, but as she goes on adventures with Rook, she realizes her full potential.
A plotline could be that Rook and Rayona are assigned to protect a special VIP from the galaxy. Along the way, they face various challenges like attacks from bounty hunters. Rayona might use her unique skills, and Rook his Plumber training. They start off not getting along well but as the story progresses, they come to respect and rely on each other. Another idea is that they discover a hidden civilization that is in danger of being destroyed by a natural disaster. They then have to figure out a way to save this civilization while also dealing with their own personal issues.
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.
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.
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.
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.
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.
First, understand the characters of Ben and Rook well. Their personalities should shine through in the fanfic. For example, Ben's more impulsive nature and Rook's calmness. Then, use the tickling concept in a creative way, like making it a unique form of communication between them.
Well, in some fanfics, there could be a plot where an alien device causes Ben to have uncontrollable tickling powers and Rook is the main target. This creates a series of comical situations as Rook tries to avoid or deal with it. Also, a plot could involve the two of them getting into a tickle fight as a form of bonding after a particularly tough battle.