Aang would also be a great focus. His carefree yet responsible nature could contrast or complement Ben's more adventurous and sometimes impulsive behavior. They could learn from each other, like Aang teaching Ben about the spiritual side of the Avatar world and Ben showing Aang some of his cool alien tech.
Zuko could be an interesting character to focus on. His complex backstory and his journey of redemption could be further explored in the context of a crossover. Maybe he could be initially suspicious of Ben and his alien powers, but then come to see him as an ally in the fight against a new threat that combines the worst of the Fire Nation's aggression and some of the alien menaces from the Ben 10 universe.
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.
I think Naruto, Aang, and Zuko would be great characters to focus on. Naruto and Aang for the reasons mentioned before, but Zuko adds an interesting element. His journey from a villain - like figure to a hero in the Avatar world could contrast with Naruto's own journey of being an outcast turned hero. They could all have great character development as they interact and face challenges together.
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.
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.
Definitely Wolverine and Spiderman. Wolverine's rough, no - nonsense attitude would contrast sharply with Spiderman's quippy and more light - hearted nature. Their fight scenes together would be amazing, with Wolverine's claws against Spiderman's webs. And they could also have some really interesting conversations about loyalty, as both are very loyal to their causes.
Naruto and Aang would be a great pair. Aang's carefree and spiritual nature could contrast with Naruto's more boisterous and determined personality. They could learn a lot from each other. Naruto could teach Aang about never giving up on your dreams in a more direct and aggressive way, while Aang could show Naruto how to be more in tune with the spiritual side of things.
Perhaps Ben, while exploring a new planet in his travels, stumbles upon a world that has a very Avatar - like culture and power system. He has to learn to use their ways in combination with his alien forms to survive. For example, using Heatblast's heat powers in a more refined way similar to firebending.
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 Deacon could be an interesting character for a 12 Monkeys fanfic. He has his own set of beliefs and goals that often clash with the main characters. A fanfic could explore his backstory and what led him to be the way he is. Also, the Pallid Man is quite intriguing. His appearance and his role in the events of the series give a lot of room for creativity in a fanfic. You could write about his relationship with the other villains or his own plans that we might not have fully understood in the original show.
Sans is an obvious choice. His laid - back yet perceptive nature would make his interactions with the FNAF animatronics fascinating. He might be able to see through their mechanical exteriors to the tortured souls within. And his ability to teleport and use his 'Gaster Blasters' could play an interesting role when dealing with the animatronics' attacks.
Ben's XLR8 and Beast Boy would be a great combination. XLR8's super - speed could be used in some really creative ways when paired with Beast Boy's ability to transform into different animals. For example, Beast Boy could transform into a bird and XLR8 could carry him at high speed, allowing them to scout out areas quickly or surprise enemies. Also, their personalities are both kind of fun - loving and energetic, so they would get along well and have a lot of banter back and forth as they fight together against the bad guys.