To start writing a Monster Hunter World crossover fanfiction, you need to have a clear vision of what you want to achieve. If you're crossing over with, say, a fantasy novel world, understand the power dynamics and the cultures in both worlds. Start by creating a character or a group of characters that will bridge the two worlds. For example, a hunter who discovers a portal to the other world. Then, build the story around their experiences. You can start small, like a simple encounter between a hunter and a character from the other world, and then gradually expand the story to include bigger events like team - ups to fight a common enemy. Also, make sure to keep the Monster Hunter World elements true to the game, like the importance of hunting gear and the behavior of the monsters.