The Doyle estate lost the case you're referring to and Klinger was allowed to publish his derivative stories.
Disney could try to sue, but I don't think their case is any stronger than the Sherlock Holmes case if the defendant is only deriving material from the original Mickey cartoon.
From the case listed "And the claim is correct, for he acknowledges that those copyrights are valid and that the only copying he wants to include in his book is copying of the Holmes and Watson characters as they appear in the earlier stories and in the novels."
Mickey Mouse will most certainly not be in the public domain anymore than he is now. Disney can also fall back on trademark law to take care of Disney's use as well.
It's a bit of a moot point in my opinion, because I don't think anyone outside of Disney would even want to make more of 1928 Mickey. It's old. It's not popular. Maybe they'd use him for a cameo or for parody, but shows already do that anyway, regardless of the copyright [1].