Another interesting aspect is the world - building. Writers often have to find ways to blend the two distinct worlds of RWBY and Fate Stay Night. They might create new locations that are a mix of the two, or they might introduce Fate Stay Night concepts like the Mage's Association into the RWBY world. This can lead to really creative and unique story settings that you wouldn't find in either of the original works alone.
The most interesting part is the combination of different powers. In RWBY, we have the unique Semblances and in Fate Stay Night, there are powerful Servant abilities. When these are combined in a crossover fanfiction, it creates really exciting battle scenarios.
The character interactions are also fascinating. Seeing how the characters from RWBY like Ruby or Weiss interact with the likes of Saber or Archer from Fate Stay Night can be really cool. They come from different worlds with different values and goals, and seeing how they either clash or cooperate in a fanfic is a lot of fun. For example, Ruby's innocent and determined nature might contrast with the more stoic and noble Saber, leading to some interesting dialogues and story developments.