One possible plot could be that there is a rift in the multiverse that brings Goku and Luffy together in a strange new world. They initially misunderstand each other and start to fight. As the fight progresses, they both realize that there is a greater threat at hand that is causing the rift. So, they team up, with Goku teaching Luffy some of his combat techniques and Luffy showing Goku the value of friendship and loyalty in a different way.
Goku in Ultra Instinct mode is extremely powerful. He has superhuman speed, strength, and can dodge attacks effortlessly. However, Luffy also has his own set of amazing abilities like his Gear Fourth form which gives him enhanced speed and power. But I think Goku would win because his combat skills and power level are just on a different scale compared to Luffy.
Maybe there's a food competition in the crossover. The chefs in the Naruto world, like Ayame from Ichiraku's, compete against Komatsu. Naruto and his ninja friends have to protect the competition from villains who want to steal the secret recipes. Meanwhile, Toriko is there to judge the food using his unique skills.
It could be Naruto in his Sage of Six Paths mode. He has incredible power, with the ability to fly, use various powerful jutsus, and his chakra is off the charts. In a fight against the characters from Toriko, his speed and strength would be very difficult to match.
One idea could be that Luffy and Toriko have a cooking competition using the strange ingredients from both their worlds. Another could be a joint adventure where they search for a mythical ingredient that is said to grant eternal strength and is hidden on an island in the Grand Line. Or perhaps a situation where the villains from both universes team up, forcing Luffy and Toriko to combine their forces to defeat them.
In a One Piece Toriko crossover, the interactions would be quite diverse. For instance, Chopper would be fascinated by the Gourmet Cells concept in Toriko's world and might try to study it. Nami could use her navigation skills to help Toriko find rare ingredients in uncharted areas. Usopp would likely be in awe of the large and dangerous creatures in Toriko's world and might come up with new gadgets to deal with them. And Luffy and Toriko would bond over their love for adventure and food. They would also team up in battles, with Luffy's elasticity and Toriko's strength making them a formidable duo against common enemies.
One common theme is friendship. Since all these characters value their friends highly in their original stories, in fanfiction they often form strong bonds across their different universes. For example, they might fight together against a threat that endangers all their worlds.
Trunks is another good option. His time - traveling experience and sword - fighting skills could add a new dimension to the crossover. He and Xeno Goku could work together to prevent disasters in different timelines.