Not a lawyer, so don't use what I say and expect me to defend you. I don't think Mickey Mouse could enter the public domain. It is trade-marked by Disney, and as long as Disney protects the trade mark, it will be private. Various films, however, like Steamboat Willie, are under copyright. read more
All works published prior to 1922 immediately entered the public domain; all works published after 1922 (including Mickey Mouse) were entitled to the full 75 years of protection. Just like that, Mickey Mouse extended his copyright death 19 years -- from 1984 to 2003. read more