Rent based storage mechanisms other than charging account maintenance fee

“Account Eviction and Recaching” section of paper states that an account can expire based on the calculated logical expiry time by VM.

The VM is free to apply any deterministic approach to determine the expiration time. One example of such a policy would be to charge a fee denominated in Libra coins that is based on the amount of time the account is stored and its size

Since storage is not free what other rent-based storage mechanisms can be considered other than charging account maintenance fee?