A good question, though we're currently living in a world that is the author's death plus 95 years in the US, so this is already standard practice. Works not covered this way get a ridiculous 120 years copyright coverage. I would say a flat 35 years from publication or 50 years for unpublished works would be reasonable.
This encourages gaming the system: companies will seek to designate as the copyright holder the youngest available employee, to make the term last longer.
