One idea I saw out there that I like a lot is to have a modest fixed copyright term on created works by default, but allow the copyright holder to pay to extend the term when it's about to expire. A payment that grows exponentially with each term such that it becomes quickly infeasible to continue copyright protection on unprofitable works while making it impossible to sustain forever on even those most profitable.

My twist: reinvest those renewal fees in the arts. Then new artists have financial support and education and publishers sitting on copyrights still make a sizable contribution to the creation of new works.