I like the idea I heard about taxing based on the owner's view of value.

Give 14 years free.

Every year after that, the copyright holder has to tell you how much they think the work is worth to them. Then you tax them some (smallish) percentage of that.

Or, you can run some public fund-raiser to raise the amount of money they said it was worth, pay off the copyright holder, and then the work is in the public domain.