Consider things like movie rights. It's not unusual for there to be a very long time lag between print publication and the release of a movie version (e.g., Watchmen or Hitchhiker's guide). I've always liked the idea of setting copyright to expire n years after the expiration of the copyright owner (where n is long enough to handle the "take care of my family when I die" and "don't disincentivize publishing stuff made by older people" issues). n = 20 is my arbitrary proposal. Copyright owned by deathless entities (corporations and such) maxes out at another arbitrary number (I like 40) and there are no options for renewals in either situation.