Which copyrighted work would society most benefit if it were to become public domain?
Likely nonfiction. Measure citations in academic databases?
Or look at the price the copyrighted work is commanding. Textbooks?
Use eminent domain for the public to buy the copyright at market price. Done correctly, this preserves creator incentives.
Is piracy economically optimally choosing what to pirate? Though, often one cannot create commercial derivative works from pirated content as one can for public domain content. But one can for academic follow-up work. (Aaron Swartz and JSTOR.)