Spectronics, a company that focuses on Inclusive Learning Technologies, has put together this list of apps for literacy support. Not all of them are free, but they make great suggestions.
This list comes from Teacher Reboot Camp. The suggested apps provide multiple ways for students to interact with various aspects of stories: plot, characters, setting, etc.
Create comic books and illustrations that include monsters and superheroes. The images and drawing tools can be used for a variety of literacy & numeracy activities as well as storytelling.
This is a great article about how iPads can be used with the writing process. It highlights best practices that teachers can follow in order to best leverage the technology, and suggests apps to support the process.