The plot is crucial. It should be engaging and make sense in the context of both franchises. Let's say it's a Transformers and 'Power Rangers' crossover. The story could revolve around a common threat that both groups have to face together. The plot should build up the tension, have some twists, and lead to a satisfying conclusion.
A great Transformers crossover fanfiction often has well - developed characters. For example, if it's a crossover with 'Back to the Future', the writer should do justice to both the Transformers and the 'Back to the Future' characters. They need to keep their personalities intact while also showing how they interact in this new, crossover universe.
Good writing style also matters. A great fanfiction uses vivid descriptions, whether it's describing the epic battles between the Transformers and their new crossover counterparts, or the emotional moments between the characters. It should be able to draw the reader in and keep them hooked from start to finish. Also, the writer should be respectful of the source materials and not deviate too much from the established lore of both franchises unless they have a really good reason for it.