This notion of copyright has basically destroyed the idea of folklore in the last few hundred years, relegating it from one of the main areas of human expression to a corner of the internet.

I think even 20 + 20 copyright is problematic. People should be free to use characters and stories while they are still relevant, and create their own variations that may even become more popular than the 'original creation' (in quotes because so much of creativity rests on the shoulder of others).

1. Copyright that's not assigned value defaults to the public domain; this provides an avenue for copyright to devolve to public domain immediately which does not currently exist;

2. Copyright that's no longer maintained by its owner devolves to the public domain (similar to 1), taking it out of "copyright limbo";

3. Any group with sufficient resources can take a copyright into the public domain as a public good, e.g., "we" could buy Wolverine into the public domain;

4. Copyright can be maintained for very long durations for entities that choose to do so;

5. The ever-increasing cost means that eventually the copyright "buys itself out", that is, when the COST is assessed at 100%, the Fed has sufficient funds to simply buy the copyright outright (this is a choice, btw, the Fed could happily accept 130% of COST!); and,

6. Absurd copyright lengths no longer universally apply.

I think (but I've not completely thought this through) that such a system could help with patent litigation. Specifically, consortia of entities can buy out a patent if the patent is weaponized. Mind you, the patent holder can no longer "squat" on a patent for free: they must pay the COST of a patent portfolio! This naturally handles the asymmetry of costs between NPEs and regular old entities.

If you like 100 years of ownership, set the rate at 1% per year, calculated on the copyright/patent holder's self-assessed price. Every century, the polity collects 100% of the value. What the polity does with the money collected is up to the polity.