An adversarial approach would also be interesting: People could open positions of "I would buy a right to use this copyright for $XYZ if it was released today"

So the copyright holder would have the option to EITHER cashout at any point (and consider the work/invested effort paid) OR counter-bid the sum of everyone to keep it.

Not sure about the implications, but it would encourage the most (economically) productive route

I'm a big proponent of compulsory licensing, which could certainly be limited to renewals so that creative control is still granted for some amount of time.