One idea could be that Iron Man somehow gets transported to the Naruto universe. He could use his technology to help the ninja in battles, like creating advanced weapons for them. Maybe he'd also try to understand chakra and combine it with his tech.
In an 'ironman naruto fanfiction', Iron Man could be mistaken for a powerful new type of ninja. He then has to prove himself in the ninja world. He might start by showing off his suit's abilities in a ninja tournament and later use his skills to protect a village from a powerful enemy.
Perhaps Iron Man teams up with Naruto to fight a common enemy. Iron Man could teach Naruto about modern technology while Naruto shows Iron Man the power of ninja skills. They could create a new kind of jutsu that involves Iron Man's suit technology.
One interesting story could be about Spiderman and Ironman teaming up to fight a new, powerful villain that neither could defeat alone. They combine their tech and skills in unique ways, like Spiderman using Ironman's nano - tech to enhance his web - shooters.
The interaction between Iron Man and Kakashi would be really interesting. Kakashi is a very intelligent and experienced ninja, and Iron Man is a genius inventor. They could exchange ideas about combat strategies and knowledge from their different worlds.
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.
Naruto would probably be super excited about the Ironman concept. He'd be constantly thinking of ways to use Ironman's gadgets in his ninja world. Yugito might be more reserved but interested. She could help Naruto see how to adapt Ironman's ideas to fit the ninja culture better. For example, making the Ironman suit more stealthy like a ninja.
One possible plot could be that Tony Stark, while testing some new interdimensional tech, accidentally ends up in the Naruto universe. He could use his advanced technology to help the ninja in various ways, like creating new weapons or defenses against the Akatsuki. Maybe he teams up with Naruto and they combine their powers to defeat a common enemy, such as a new super - powerful villain that neither world could handle alone.
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.