Poison Ivy herself is an extremely interesting character to focus on in this crossover. Her complex relationship with nature and her rather extreme views on protecting plants could create a lot of tension and drama. She might see Ben 10's alien forms as either threats or potential allies in her fight to preserve the natural world. Her powers are also very unique. She can control plants and use them in a variety of ways, from attacking enemies to creating elaborate defenses. In a fanfiction, exploring how she reacts to Ben 10's world of aliens and advanced technology would be fascinating. For example, how would she deal with the idea of an alien plant that has completely different biological rules than the plants on Earth?
It could be Ben himself. Imagine him using his alien forms in the Naruto universe. His Heatblast form could be really useful against ice - based jutsu, and Four Arms could take on some of the physically strong ninja. He would bring a whole new element of power and chaos to the Naruto world, and his reactions to the different cultures and powers in Naruto's world would be interesting to explore.
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.
I think Ben's Heatblast and Thor would be an interesting combination. Heatblast can generate intense heat and fire, while Thor has his lightning powers. They could create some really powerful elemental attacks together.
Definitely Ichigo and Ben. Ichigo has his strong sense of justice and his powerful Hollow - Soul Reaper abilities. Ben, on the other hand, has the Omnitrix with all its different alien forms. They would make an interesting duo as they could learn from each other's fighting styles and powers. Ben could learn about the spiritual aspect of Ichigo's powers, and Ichigo could be amazed by the physical transformations Ben can do.
Ben and Ruby would be an interesting pairing. Ruby is young and full of energy, always eager to learn and improve. Ben, with his experience using the Omnitrix, could teach her a lot about adapting to different situations. And Ruby could show Ben the importance of teamwork in her own unique way.
Ax from the Animorphs would also be a great focus. He's an alien himself, so his perspective on Ben and his powers would be unique. He could compare and contrast his own Andalite abilities with Ben's Omnitrix powers. Plus, Ax is really intelligent and his knowledge of different species and technology could add a lot to the crossover story.
I think it would be Ben. His ability to transform into various aliens brings a lot of possibilities. He can be a tough fighter like Rath or a strategic thinker like Grey Matter. In the crossover, seeing how he interacts with the Gems and Steven would be really fascinating.
A cool plot could be that Ben 10's Omnitrix malfunctions near Poison Ivy. This causes some of his alien DNA to mix with her plant - based essence. As a result, they both gain new and strange powers. They then have to figure out how to control these powers while also dealing with villains who want to use this new situation for their own evil plans. Or, there could be a story where Poison Ivy is initially suspicious of Ben 10 because of his alien nature. But when she sees how he respects nature in his own way (through his alien forms' relationships with different environments), she warms up to him. Together, they could go on a journey to save a forest that is being destroyed by a powerful and greedy corporation. Ben 10 uses his strength and agility as different aliens, and Poison Ivy uses her plants to slow down the corporation's machines and workers.
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.
Ben as Diamondhead and Zoro would be really interesting. Diamondhead's crystal - like body could provide a great defense against attacks, and Zoro's swordsmanship could be enhanced with Diamondhead's sharp edges. They could team up in battles and create some unique combat strategies.