One possible plot could be that in a world where Naruto and Kamen Rider exist together, Naruto, for some reason, sees the Kamen Rider as a threat to his village's safety. Maybe the Kamen Rider's powers are misconstrued as dark magic. So Naruto, in his role as Hokage or a high - ranking shinobi, banishes the Kamen Rider. This could lead to the Kamen Rider having to prove his innocence and find a way back to the village to clear his name.
The shinobi in Naruto's village would likely be divided. Some would support Naruto's decision wholeheartedly, believing that any unknown power like the Kamen Rider's is dangerous. Others might be more skeptical, especially those who are more open - minded or have interacted with the Kamen Rider. As for the Kamen Rider himself, he would probably be angry and confused at first, but then determined to prove his worth and return.
First, you need to study both universes well. Know the powers, personalities, and backstories of the characters from Kamen Rider Gaim and Naruto. Then, come up with an interesting concept for the crossover. It could be something like a portal opening between the two worlds.
First, you need to understand the basics of both universes. Know the characters, their powers, and the settings. Then, think of an interesting premise like a Kamen Rider arriving in the Naruto world. Next, plan out your plot. Decide how the characters will interact and what challenges they'll face.
One idea could be a story where a character has a bloodline that combines the powers of Naruto's ninja world and Kamen Rider's technology. For example, they could have the Sharingan-like ability to analyze Kamen Rider powers and use them more effectively. This would create a unique dynamic in battles.
I really like 'Kabuto in the Hidden Leaf'. In this fanfiction, Kamen Rider Kabuto's character is inserted into the Naruto universe. He has to deal with the politics and challenges of the ninja villages. The author does a great job of showing how Kabuto's powers and his way of thinking contrast with the traditional ninja ways. It's a fresh take on both franchises.
A possible fanfiction plot could involve the villains from both universes teaming up. Naruto and Kamen Rider Wizard would then have to put aside their differences and work together to defeat this combined evil force. They could learn from each other's fighting styles during the process.
One common theme is the power exchange. For example, a Kamen Rider might teach Naruto or other ninja about their technology - based powers, and in return, the ninja teach the Kamen Rider about chakra and ninjutsu. It's interesting to see how they combine these different power systems.
One interesting fanfiction could be where the characters from Kamen Rider Gaim end up in the Naruto world. Maybe they team up with the ninja to fight a common enemy. For example, they could use their unique Rider powers along with ninja techniques.
You could check out fanfiction.net. There are often some really creative crossovers there. I've found some that blend Naruto's ninja world with Kamen Rider Faiz's unique hero concept quite well. They play with the different powers and character dynamics in interesting ways.