One idea could be a Naruto and Harry Potter crossover. Picture Naruto arriving at Hogwarts, his ninja skills and chakra creating chaos among the wizards. Another could be a Naruto and One Piece crossover, where Naruto's team sails with the Straw Hats, facing off against the Akatsuki and the Marines. And a Naruto and Bleach crossover, with Naruto and Ichigo teaming up to fight Hollows and rogue ninjas.
For naruto crosover fanfiction, consider a crossover with the Pokémon world. Naruto and his friends could catch Pokémon and use them in battles, while also using their ninja abilities. A Naruto and Digimon crossover would also be interesting. The ninjas could enter the Digital World and help the Digimon fight against evil. And a crossover with the Gundam universe, where Naruto's skills are put to the test in mecha battles.
To begin a naruto crosover fanfiction, start by choosing a crossover that really interests you. If it's a Naruto and Lord of the Rings crossover, for instance, you need to understand both universes well. Read up on the lore of both. Then, find a common ground for the story. Maybe Naruto is sent to Middle - earth to help fight Sauron. Develop your main characters' roles in this new setting. You can start with a simple scene, like Naruto arriving in Middle - earth and being confused by the new environment, and then expand from there.
One common theme is the exploration of relationships between male characters from Naruto and the other crossover series. It often delves into their emotional bonds and how they develop in a romantic or sexual context in the yaoi aspect.
I'm not sure which specific 'naruto crosover yaoi fanfic' you would like. But if you search on platforms like FanFiction.net, you can filter the results by 'Naruto' and 'Yaoi' and see what crossovers are available. Maybe there are some really interesting ones that cross Naruto with other popular shonen or seinen series in a yaoi - themed way.
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.
You could also write a fanfiction about Naruto being raised by a different clan in the village. For example, if he was raised by the Uchiha clan, it would have a huge impact on his character development and relationships within the village.