I think that's a great idea. The value of an IP could be self-assessed, with the caveat that any party has an option to purchase the IP for e.g. 125% of the self-assessed value.

The tax on this value could scale from 1% for year 10 onwards, increasing by 0.25% per year. Year 50 of your copyright? Be prepared to pay 11% APR rent on its value.