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.
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.
One interesting fanfic could be where the Kamen Riders from Build end up in the RWBY universe and team up with the huntsmen and huntresses to fight a common enemy. For example, they might face off against a Grimm that has been enhanced by some Evolt - like technology.
Often, they start off as strangers. For example, the Kamen Rider Build characters might accidentally end up in the RWBY world. They'd be confused at first, but then start to interact. Maybe the RWBY characters would be cautious at first, like Weiss might be skeptical of these new 'visitors' until she sees their combat abilities.
You can start by checking out some popular fanfiction websites like Archive of Our Own (AO3). It has a wide range of Kamen Rider Build fanfictions written by fans from all over the world. Another option could be FanFiction.net, which also has a significant collection of such works. Just search for 'Kamen Rider Build' on these platforms and you'll find a lot to choose from.
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.
To create an engaging Kamen Rider Build OC fanfic, first, develop a unique and interesting original character (OC). Think about their backstory, personality, and how they fit into the Build universe. For example, your OC could be a scientist with a special ability related to the Build technology. Then, plan a plot that involves your OC in the main events of the Build story. Maybe they help Build in a crucial battle or have their own side quest that impacts the overall plot.
One key element is a well - developed OC. This includes their appearance, like what kind of suit or costume they have if they are a rider - type character. Their personality is also crucial, whether they are brave, cunning, or kind - hearted. Another element is the plot. It should have a clear beginning, middle, and end. For example, the OC could start as an unknown figure and gradually become a key player in the fight against the evil in the Build world.
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.
You can start by looking on fanfiction websites like Archive of Our Own (AO3). It has a large collection of fanfictions from various fandoms, including 'Kamen Rider Build'. There are different categories and tags that can help you find the type of fanfic you're interested in.