Definitely Ben and Ash. Ben with his Omnitrix and ability to transform into different aliens brings a whole new dimension of power and adventure. Ash, on the other hand, has his experience and deep connection with Pokemon. Their interactions would be fascinating. For instance, Ben might be amazed at how Ash can communicate with his Pokemon without any high - tech device, and Ash could be in awe of Ben's alien forms and their powers.
I think Pikachu and one of Ben's most used alien forms like Four Arms would be interesting. Pikachu is so iconic in the Pokemon world, and Four Arms is strong and has a unique personality. Imagine the battles they could get into together. Also, Pikachu's electric attacks could interact in unexpected ways with Four Arms' brute strength. And if they faced a common enemy, they would have to learn to work together despite their very different natures.
Professor Oak and Grandpa Max would be great characters to focus on. Professor Oak has his vast knowledge of Pokemon, while Grandpa Max has years of experience dealing with aliens and otherworldly threats. They could combine their knowledge to figure out how to handle the situation in a crossover. Maybe they start researching the similarities between Pokemon and the aliens from Ben's world, and their discussions could lead to some really interesting plot developments, like finding a way to use Pokemon powers to enhance the Omnitrix or vice versa.
Vilgax from Ben 10 could be an interesting character to include. If he somehow gets his hands on a Green Lantern ring or the technology related to it, he could become an even more formidable foe. This would force Ben and the Green Lanterns to team up against a common enemy who has combined the power of both universes in a dangerous 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.
Definitely Ben Tennyson himself. He has a whole range of alien forms and abilities, and seeing how he would adapt to the Avatar world would be really cool. For instance, how would he use Four Arms in a fight against earthbenders or how would he use XLR8's speed in a chase through the air temples.
Naruto would be the most interesting. His energetic and head - strong personality would make for great interactions with the Pokemon world. He could be constantly amazed by the different Pokemon and their abilities, yet still use his ninja skills and quick thinking to adapt. His ability to make friends easily would also mean he'd quickly bond with his Pokemon and other trainers.
Ash and Four Arms would be a great pair. Four Arms is all about brute strength, and Ash has his knowledge of Pokemon and his own battle skills. They could work together in battles against powerful foes. Maybe Ash could learn some new fighting techniques from Four Arms, and Four Arms could learn about the subtleties of Pokemon battles. It would be a clash of two different but equally cool fighting styles.
Mewtwo could be a very interesting character in this crossover. Mewtwo is a powerful and intelligent Pokémon. In a fanfiction, it could be intrigued by Ben's Omnitrix and the concept of being able to transform into different beings. Mewtwo might initially see Ben as a threat or a potential ally in its own quest for power or understanding of the world. This could lead to some intense and thought - provoking interactions between Mewtwo and Ben.
Batman and Black Widow would be really interesting. Batman is all about mystery and detective work in the shadows, while Black Widow is a master spy. They could learn a lot from each other's skills in stealth and intelligence gathering. Also, Thor and Batman could be a great focus. Thor with his god - like powers and Batman with his human ingenuity. Imagine Batman trying to figure out the science behind Thor's hammer while Thor is amused by Batman's determination in the face of the supernatural.
Definitely Naruto and Geralt. Naruto with his bright - eyed optimism and powerful ninja skills would contrast greatly with Geralt's brooding and more practical monster - hunting skills. Their interactions would be full of learning and cultural clashes. For example, Naruto might be shocked at how Geralt deals with certain 'monsters' that in Naruto's world could potentially be redeemed. And Geralt would be intrigued by Naruto's chakra and jutsu.
Definitely Naruto and Link. Naruto's energetic and determined personality would contrast well with Link's more stoic nature. They could learn a lot from each other. Also, Zelda would be interesting. Her wisdom and magical abilities would add a different dimension. And Sasuke could also be a great character to include. His dark past and thirst for power could lead to some interesting interactions in the Zelda world, especially when faced with the power of the Triforce.
Definitely Naruto and the main Guyver character. Naruto because of his determination and growth throughout the Naruto series. He would be really interesting to see how he adapts to the Guyver's powers. And the Guyver, because its powers are so unique and different from the ninja abilities in Naruto. Their interactions could be full of misunderstandings at first as they come from different worlds, but then they could learn from each other.