Goku and Luffy would be really interesting. Goku is all about pure strength and the pursuit of becoming stronger through training, while Luffy is more about the adventure and his dreams. Their different outlooks could lead to some great conversations and also some funny moments, like when Luffy tries to eat Goku's senzu beans thinking they are some kind of food from his world. Another interesting interaction could be between Vegeta and Zoro. Vegeta is very proud and serious, and Zoro is a swordsman with his own code of honor. They could have some intense sparring sessions and also respect each other's strength.
One of the most interesting interactions would be between Krillin and Usopp. Krillin is often the underdog in the Dragon Ball Z world, but he has his own skills and courage. Usopp is also an underdog in the One Piece crew, known for his lies that sometimes turn into reality. They could bond over their insecurities and also come up with some creative strategies together. Also, Bulma and Franky could have a great interaction. Bulma is a brilliant inventor, and Franky is a cyborg with his own amazing inventions. They could collaborate on some new gadgets or even argue about whose inventions are better.
I think Goku and Nami would have an interesting interaction. Nami is very smart when it comes to money and navigation, and Goku is kind of clueless about those things. She could be constantly frustrated with Goku's naivete in financial matters. Also, Piccolo and Brook would be cool to see together. Piccolo is a Namekian with his own unique biology and powers, and Brook is a skeleton with his musical abilities and his own form of immortality. They could share stories about their different existences and maybe even learn something from each other.