If their worlds were to merge, it could start with the exchange of ideas. Maybe a scientist from Build's world is interested in the concept of chakra and travels to the Naruto world to study it. Meanwhile, a ninja from Naruto's world is intrigued by Build's technology. As more and more people start to interact between the two worlds, the boundaries start to blur. Eventually, the two worlds merge completely. There would be new laws and social norms as a result of this merger, and new threats and challenges that require the combined efforts of both Naruto and Kamen Rider Build's characters to overcome.
The worlds could merge through a mysterious portal. This portal could be created by some powerful jutsu in Naruto's world that goes wrong or by an experiment in Build's world. Once the worlds start to merge, the characters would be in for a shock. The ninjas in Naruto's world would see the advanced technology of Build's world, and the people in Build's world would be amazed by the use of chakra.
In fanfiction, the merge could happen because of a powerful jutsu gone wrong. This jutsu could distort space - time and connect the two universes. In the merged world, the Gundam pilots would have to deal with the threat of ninja attacks. They could use their Gundam building skills to create mechas that are better equipped to handle ninja abilities like genjutsu. Meanwhile, the Naruto characters would be fascinated by the Gundam battles and might try to participate using their own combat skills.
One great 'Kamen Rider Build' fanfiction story could be about an alternate universe where Build teams up with other Kamen Riders from different series to fight a common super - powerful enemy. This kind of crossover fanfic can be really exciting as it combines different powers and personalities.
The characters would likely start off with some level of mistrust. For instance, the Avengers might be suspicious of Build's powers and his motives at first. However, as they face threats together, they would start to respect each other. Iron Man and Build could bond over their love for technology and inventing new gadgets. Captain America might be initially skeptical but would come to admire Build's courage in battle.
In a Naruto and Batman crossover fanfiction, their worlds could merge in a few ways. One way could be through a portal. Maybe some sort of evil magic or advanced technology creates a portal between Konoha and Gotham. This would bring Naruto and his friends into Batman's world, or vice versa. Another possibility is that a powerful villain has the ability to manipulate space - time and decides to bring the two worlds together for his own evil purposes.
The bright and colorful world of RWBY with its various kingdoms and unique landscapes could contrast with the more technological and urban - like settings often seen in Kamen Rider Gaim. But they could blend in a way that the advanced technology of Gaim is incorporated into RWBY's world, like Gaim's devices being used to enhance Dust - based weapons.
Well, in a Naruto and Kamen Rider Build fanfiction, we could see a situation where the Kamen Rider Build's transformation devices are scattered across the Naruto world. Naruto and his friends go on a quest to find these devices. Along the way, they encounter different challenges and enemies. As they find the parts, they start to understand how Build's powers work. They might even accidentally trigger some transformations. And once they fully assemble the device, they have to decide who will use it to fight the upcoming big threat that is attracted by the presence of Build's technology in their world.
One idea could be that the Avengers are transported to the world of Kamen Rider Build. They team up with Build to fight a common enemy that has the power to disrupt both universes. For example, a villain who can manipulate matter like Build's powers but on a much larger scale, threatening Earth in the Avengers' world and Build's city in his world.
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.
Naruto's ability to sense chakra could be used to detect hidden enemies in the Kamen Rider Drive world. In that world, there might be villains who can cloak themselves using technology. But Naruto's chakra - sensing ability would cut through that and allow him and Drive to be one step ahead in battles. And Drive could use his gadgets to then counter - attack these enemies.
In some fanfictions, the merge happens when a powerful artifact from either universe is activated. This could be a Kamen Rider's belt or a powerful relic from Highschool DxD. When this happens, it creates a rift that pulls elements from one universe into the other. The characters then have to deal with the new and unexpected situations. For instance, the devils in DxD might be intrigued by the Kamen Rider technology and try to harness it for their own purposes.