There are two competing interests here. First, Disney wants to keep control of Mickey Mouse cartoons, and it doesn't feel unreasonable at all that they should. Second, the rest of the stuff that is of little commercial value but of potential cultural value should be freed from uncertain or uninterested ownership.

The answer feels trivial to me, which means I'm probably missing something. Fixed term for a few decades from the creation of the work, maybe 30 years or 40 years. After that, you need to give a copy of the work to the Library of Congress, pay a nominal fee, and you get a 5 year extension. Take as many of those as you want.

This greatly simplifies everything. If you find a work that is older than the base term, you look it up in the copyright extension database. If it's in there, you can find out who owns the rights to it, and try to license rights. If it isn't in there, it's public domain. If somebody doesn't stay on the ball enough to renew their rights every five years, it probably isn't all that commercially valuable, and moves into the public domain.