[eru]

You could just make it more and more expensive to extend copyright.

Ie first twenty years are free, the n-th year after will cost you some multiple of n USD. (Or make it grow quadratic or exponential.)

Or you can get a fair valuation pretty easily: let Disney post a value for Mickey Mouse they'd want to be taxed on. Lower value means less tax. But the self-declared value comes with the obligation to sell to any comer who offers that much in cash.

[valleyer]

You don't even have to force them to license it. If they specify a low value, then the damages in the copyright lawsuit should be commensurately lower.

[eru]

Leaving it to the courts is an interesting idea.