The public domain version of the character doesn’t include significant design changes made in later works, like Sorcerer’s Apprentice Mickey from Fantasia in 1940. And you can’t produce a work that falsely represents itself as a Disney production or a piece of official merchandise, since Mickey Mouse is also a registered Disney trademark.
The version of the character that enters the public domain is only the one from Steamboat Willie. What exactly constitutes that version of the character versus a more modern one is an issue that will have to be left to Disney’s lawyers and the courts to decide.
The public domain is the final destination of any copyrighted work — it’s part of a compromise that acknowledges the benefits of letting artists and thinkers control and profit from their work in the short term while freely building on each other’s ideas in the long term, a balance Disney itself relied upon when making fairy-tale adaptations like Snow White and Cinderella.