I've thought about this quite a bit. I think one fair approach would be to have a 2 tiered copyright release, where there's still a pretty long time before a work lapses into the public domain, but before that, a relatively short amount of time after the release of the work (say, 10-15 years) the original owner loses the power to dictate who uses their work. Basically, between that point and the public domain lapse, anyone would have the option to accept a default contract of paying the creator some standardized % royalties. The creator would still have the option to accept explicit contracts with weaker terms, for example waiving the royalties for projects they want to give away.