A more complex idea would be to have a fanfiction set in a post - war scenario in both worlds. Naruto has finally achieved peace in his world, and Toriko has explored most of his world's ingredients. They meet through some sort of interdimensional rift. Together, they start a new journey to explore other dimensions in search of new things, be it new forms of power or unique ingredients.
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.
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.
The first implication would be a change in Naruto's powers. Toriko's world is centered around food and the unique abilities related to it. Naruto might inherit some of these food - based powers, like being able to enhance his strength by consuming certain rare foods. This would be a huge departure from his normal chakra - based abilities in the Naruto universe.
Definitely Naruto and Toriko themselves. They are the main characters of their respective series, so having them interact is a must in most fanfictions.
One interesting idea could be the combination of Naruto's ninja skills with mecha technology. For example, Naruto might use his chakra to power up the mecha in unique ways. Another idea could be the new enemies that are created specifically for this mecha - Naruto concept, like robotic shinobi that require different combat strategies compared to normal foes. Also, the design of the mecha Naruto itself can be really cool, perhaps with features inspired by the Nine - Tails.
Another idea is that Aizen somehow ends up in the Naruto universe during the time of the Fourth Great Ninja War. He could try to manipulate both sides for his own gain. But Naruto, with his ability to see the good in people, might try to reason with Aizen and show him a different path. It could be a story of redemption or a great battle if Aizen refuses.
In a 'Naruto Mermaid Fanfiction', the mermaids could be in danger from an evil organization that wants to capture them for their precious scales. Naruto teams up with a mermaid princess to stop this threat and in the process, they learn about each other's worlds.
One idea could be a crossover where Naruto and Pucca end up in the same world due to a strange ninja jutsu gone wrong. Naruto is confused by Pucca's constant chasing of Garu, and he tries to help Garu escape in his own Naruto - style way.
Well, you could write a fanfiction where the Nine - Tails in Naruto has some sort of connection to a powerful entity in the Seraphim universe. This connection could lead to a series of events that require Naruto and the Seraphim characters to work together. Or perhaps a character from Seraphim is reincarnated as a ninja in the Naruto world and has to adapt to the new way of life while still having some of their Seraphim abilities.
One idea could be a blacksmith who creates unique ninja tools with special abilities that give the wielder an edge in battle. For example, a kunai that can change its shape during flight to avoid enemy defenses.