Satsuki Kiryuin and Ben would be really interesting. Satsuki is so calculating and in control. When she meets Ben, she would probably be intrigued by his Omnitrix and see it as a potential tool for her own plans. Ben, on the other hand, would be wary of her motives. Their interactions would be full of power plays and mind games as Ben tries to figure out if Satsuki is an ally or an enemy while Satsuki tries to manipulate Ben into doing what she wants.
A fanfic could start with Ben being sent to the Kill la Kill universe through some sort of interdimensional rift. In this new world, he discovers that the Omnitrix can absorb the power of Life Fibers. This leads to a race between the heroes and the villains of Kill la Kill to either use Ben's newfound ability for good or to take it over for evil. Ben has to learn the ways of this new world quickly while also trying to find a way back home.
I think the interaction between Ruby and Ben would be really interesting. Ruby is very energetic and has a strong sense of justice. Ben, with his experience of using the Omnitrix to save the day, could share some of his stories with her. They could also have some friendly competitions regarding their combat skills, like who can take down a group of Grimm faster, with Ruby using her Crescent Rose and Ben using one of his alien forms.
Another interesting interaction could be between Gwen (from Ben 10) and Nami (from One Piece). Gwen's intelligence and magical abilities would contrast with Nami's skills in navigation and her love for treasure. They could exchange knowledge, with Gwen teaching Nami some basic magic spells in exchange for Nami teaching Gwen about navigation techniques across the Grand Line.
The interaction between Ben and Robin would be really interesting. Robin is a great strategist and detective, while Ben is more of a brash hero who relies on his alien forms. They could learn a lot from each other. Robin could teach Ben about planning ahead and thinking through his actions, while Ben could show Robin how to be more adaptable in combat by using different alien abilities.
Another interesting interaction would be between Aqualad and Ripjaws. Aqualad has his own water - based powers and a strong sense of leadership in the underwater domain. Ripjaws, being an alien fish - like form of Ben, could bring a different perspective. They could work together to explore underwater areas or fight against water - related threats, sharing their knowledge of different aquatic environments.
The interaction between Yu Narukami from Persona 4 and Makoto Yuki from Persona 3 could be really interesting. Yu is a very calm and perceptive leader, while Makoto has his own sense of leadership. They could share their experiences in leading their respective teams and learn from each other.
Definitely Ben and Batman. Batman would be intrigued by Ben's Omnitrix and his ability to transform into different aliens. He would try to analyze it while Ben would be in awe of Batman's detective skills and gadgets. It would be a clash of two very different mindsets - one a young, impulsive hero and the other a brooding, strategic one.
One interesting crossover is when the ninjutsu from Naruto is combined with the combat style of Kill la Kill. For instance, Naruto using his shadow clone jutsu while Ryuko uses her scissor blade attacks in tandem. It creates a very dynamic and visually appealing fight sequence.
The interaction between Saitama and Levi would be really interesting. Levi is known for his serious and efficient fighting style, while Saitama is super - powerful yet non -chalant. Levi would probably be both intrigued and a bit annoyed by Saitama's laid - back attitude towards the Titans. Saitama, on the other hand, would probably just see Levi as another person in this strange world.
Superman and Thor would be really interesting. They both have superhuman strength and are sort of otherworldly figures. Thor might be a bit jealous at first of Superman's powers, but they could end up having a friendly competition to see who can lift the heaviest object or fly the fastest. Another interesting interaction could be between Superman and Iron Man. Iron Man, with his technological genius, might be eager to study Superman's powers and find a way to replicate them in some form. And Superman could be fascinated by Iron Man's high - tech gadgets.