Trademarks don't expire (well technically they do but as long as you can prove you are using them you can extend them indefinitely). What you are talking about is the Mickey Mouse copyright. And strictly speaking this only relates to Mickey Mouse and Steamboat Willie copyright. The law is a little vague on how the expiration will apply to later incarnations of Mickey Mouse. For example the Mickey Mouse in Fantasia looks significantly different than the one in Steamboat Willie. The general belief seems to be that the expiration will only mean people will be able to distribute copies of the original short, not give wholesale access to all Mickey Mouse properties. Also, Disney has trademarked the name Mickey Mouse and specific pieces of artwork depicting Micky Mouse (logos). So people cannot suddenly start selling Mickey Mouse knock offs as that would violate trademark law.