The plot should be engaging. It should have a clear goal for Kabuto in the new crossover world. Maybe he has to find a way back to his own world or he has a new mission in this combined universe. A good plot will keep you hooked from start to finish. Also, the way the author combines the elements of both worlds, like if it's a Kabuto Yakushi and One Piece crossover, how they merge the ninja abilities with the Devil Fruits and pirate world elements is crucial.
Consistency in Kabuto's character. Even though he is in a new and different world, his core traits should remain. If he was a sly and intelligent character in the original Naruto series, he should still have those qualities in the crossover fanfiction. Also, the fanfiction should explore new aspects of his character that might come up due to the crossover situation. For example, if he crosses into a world full of magic, how his medical ninjutsu might be affected or enhanced by the magic system there.