Trademark will still protect the characters Mickey and Minnie, and the names Mickey Mouse and Minnie Mouse, and prevent them from being used as part of any other company's advertising or corporate identity.
What will be in public domain are the particular graphic expressions, i.e. drawings, of Mickey and Minnie used in the cartoons and illustrations on which copyright has expired, as well as those cartoons entire. These particular graphic representations, and their particular voices, will be able to be sampled and repurposed.
No company will be able to take an illustration of Mickey that is still under copyright and use it as its own logo. No company will be able to call its product Mickey Mouse Condoms or Minnie Mouse IUDs or whatever, as that would still infringe on Disney's trademarks. But you can expect to find Steamboat Willie turning up in all kinds of places.