The message is another viewpoint on the Cinderella story, so writing a letter is perfectly fine to do that in. The new viewpoint allows for a retelling of the story through another character's eyes.
The letter closely follows the original story in a fairly accurate manner, and the incorporation of a new voice to tell the story really enhances the differentiation from the original story vs the new message.
The tone is upbeat since this is a positive letter overall, and the intended audience could be anyone, but more likely would be readers of the original Cinderella story.
The language is reflective of the original story which has very simple and easy to understand language. This is totally appropriate since the character that has become the narrator for this version is still in that fantasy world and this piece should still reflect that.
This meets the length requirements.
I like how there is a creative process which has resulted in a new character to be fleshed out, and I'm looking forward to the next objects throughout the semester. I would be hesitant to try to modernize the story but instead explore new mediums and methods to present the unaltered story in a new way.
The image piece could be drawings if you are confident enough to do that, but also could just be still frames from the movie which illustrate this letter. I'm not sure how this could be translated into a sound or video object since it is a movie to start with, but I might explore making a .gif for the video object.