The original author (not an assignee) should get N years free and should be able to renew every N years for some fee that escalates upwards with a single retroa

The original author (not an assignee) should get N years free and should be able to renew every N years for some fee that escalates upwards with a single retroactive period if you haven't paid. If you hit a second period that you haven't paid, copyright dropped. If you aren't the original author, you get to renew once for N years--no more. This is a nice compromise in that it disincentivizes simply squatting on properties--you have to pay money to maintain later copyright so you have a strong push to make money on it.