First, decide which two universes you want to cross. Then, research both universes thoroughly. For example, if you're crossing Xenoblade with Dragon Quest, know the lore, characters, and settings of both. Next, create a plot that makes sense for both sets of characters. Maybe there's a common enemy or a shared goal. Start with an interesting hook, like a character from Xenoblade suddenly appearing in the other universe. Then, just start writing and let the story flow.
To start a Xenoblade crossover fanfic, begin by choosing the crossover concept. Are you going to mix it with a post - apocalyptic world or a high - fantasy realm? Once you've decided, make a list of the main characters from both Xenoblade and the other universe. Think about how they would interact. Do they have similar values or very different ones? Then, outline a basic story arc. It could start with a mystery or a sudden event that brings the two groups together. And don't forget to give your story a unique voice and style.