I think Kagome and Kyon would have some interesting exchanges. Kyon is the voice of reason in the SOS Brigade, and Kagome coming from a different world with different rules would challenge his view of things. He might be skeptical of her at first, but as they interact more, he'd start to see that there's more to her world than he thought. And Kagome would be intrigued by Kyon's sarcastic yet practical nature.
Kagome is also a great choice. Her knowledge of the feudal era and her spiritual powers would make her stand out in Haruhi's world. She could be the one to introduce a sense of mystery and ancient wisdom to the modern and often zany world of the SOS Brigade. Also, seeing how she adapts to Haruhi's world and tries to find a way back to her own while dealing with Haruhi's wild ideas would be really engaging.
One possible plot could be that Kagome from 'Inuyasha' accidentally stumbles into the world of 'The Melancholy of Haruhi Suzumiya'. Haruhi, being Haruhi, decides to make Kagome part of her SOS Brigade. Kagome then has to deal with Haruhi's wild ideas while also trying to find a way back to her own world.
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.
I think Luffy and Spider - Man would be really interesting. Luffy with his carefree and fearless nature, and Spider - Man with his witty remarks. They would probably get along well and have some crazy adventures together.
Zoro and Vegeta would also have a great interaction. Zoro, with his strong sense of honor and his goal of becoming the world's greatest swordsman, would be a worthy opponent for Vegeta. Vegeta would be interested in Zoro's swordsmanship and might even see him as a potential rival. They could have some intense training sessions together, sharing their knowledge of combat and power - up techniques.
Definitely Joker and Makoto Yuki. Joker is the leader of the Phantom Thieves, very confident and strategic, while Makoto Yuki is a strong and silent type. Their leadership styles could clash or complement each other.
Definitely the interactions between the two versions of Leonardo. The 2012 Leo is more light - hearted in some ways compared to the very serious 2003 Leo. They could have some great moments of leadership clashes and then learning from each other. For example, the 2003 Leo could teach the 2012 Leo some of his more stoic leadership techniques.
I think the interaction between Joker (from Persona 5) and Yu (from Persona 4) would be really interesting. They are both leaders of their respective groups, so they could exchange leadership strategies and learn from each other. Also, Morgana and Teddie would be hilarious together. Morgana's serious yet sometimes clueless nature and Teddie's over - the - top and comical personality would create some great comedic moments.
Definitely Naruto and Luffy. Luffy is carefree and impulsive, while Naruto is also quite headstrong but has a strong sense of justice. Their interactions would be hilarious as they might not understand each other's worlds at first. For example, Luffy might be confused about Naruto's chakra and ninja tools, and Naruto might be shocked by Luffy's rubber body. Another interesting interaction could be between Naruto and Zoro. Zoro's serious and dedicated nature towards his swordsmanship could contrast with Naruto's more energetic and sometimes goofy nature.
Definitely Naruto and Saitama. Naruto's energetic and determined personality against Saitama's laid - back and seemingly bored attitude would create a great contrast. Naruto would be constantly amazed by Saitama's power, while Saitama would be a bit puzzled by Naruto's complex world of ninjutsu and chakra.