If someone thinks, for a second, that the Mouse Corp is going to make their many IPs public domain is, probably, dreaming in Technicolor
Legally speaking, they don't get a say in it. Kinda.
From a copyright law perspective, you get (under American law) life of the author plus 70 years (this gets a little complicated for works that were copyrighted before 1976, as I recall, but that's a separate discussion). Once the author has been dead for 70 years, that's it. The original work is in the public domain, and you can do whatever the hell you want
with that work. That includes making "derivative works", aka sequels, spinoffs, etc. It's not a question of "releasing" it to the public domain; you don't have any control over that work in the first place.
What you
do have control over are other derivative works, as well as trademark if you can apply it.
So, in the case of, say, Steamboat Willie, come January 1, 2024, you'll be able to show that whenever you want without paying Disney a dime. You can reproduce it and distribute it all you want, too. And in theory, you could make a spin-off, like Steamboat Murder, where Mickey runs around on the boat, whistling while he slices other cartoon characters in two.
That last one could be complicated, though, because Disney has incredibly strong trademarks and aggressively protects them. Basically, without diving too deeply into trademark law (Which is its own minefield), Disney could argue that your new stuff doesn't violate
copyright, but rather infringes upon their
trademarks and that's the rubric under which they're stopping you.
This, however, is only an issue because of how woven into Disney's branding Mickey Mouse is. If we were talking about, say, Sherlock Holmes. ALL of Holmes' original stories are now in the public domain, meaning anyone can do an adaptation of any of the original stories. Anyone can tell any Sherlock Holmes story they want with the character, because the character itself isn't subject to copyright protection anymore. And the new story that you tell, or the specific re-telling of an old one that you do? That version is protectible under copyright law.
But you could do Sherlock Holmes vs. Godzilla, or Sherlock Holmes meets Jules Verne, or a Sherlock Holmes/Jazz Singer mashup where Holmes sings "Yes Sir, That's My Baby" because all three works (Holmes, Jazz singer, and the song) are in the public domain.
