The interactions with the new characters from the crossover world matter a great deal. If it's a crossover with a different anime universe, say, the way Ichigo's full hollow form interacts with those characters should be interesting and believable. It could be through battles, alliances, or just simple conversations. Moreover, the pacing of the story should be right. Not too slow that it gets boring, and not too fast that the reader can't keep up.
A good 'Full Hollow Ichigo Crossover Fanfiction' often has a well - developed plot. For example, it should explain clearly how Ichigo's full hollow form ends up in the crossover world. Also, strong character development is key. The author should show how Ichigo's hollow side evolves or reacts differently in the new setting.
Another important aspect is the exploration of Ichigo's identity. In a full hollow form, he is different from his normal self. A great fanfiction will dig into how this new form affects his sense of self, his goals, and his relationships. It might also explore the moral implications of his actions in the crossover, especially when his hollow instincts come into play. This adds depth to the story and makes it more than just an action - packed crossover.