Yes, I planned to write about the parking system later, but since you ask I'll just cover it here.
In my view, interest rates should be universal across nubits. The parking system destabilizes nubits, serves no useful purpose, and should be abolished.
Parking creates a form of asset-liability mismatch, known as maturity mismatch or term mismatch. Term mismatch plays an important role in the collapse of fixed exchange rate regimes, e.g. the Asian Financial Crisis.
The value of assets in the nubits system are predictable in the short-term, but highly unpredictable over a longer time horizon. The debt structure of Nubits should match its asset structure.
Nubits should only issue short-term debt. The option of purchasing long-term debt by parking nubits should be taken off the table.
Since negative interest rates encourage Nubits holders to sell their Nubits for USD at parity now, they effectively allow shareholders to exercise a call option on short-term debt commitments. Prudent exercise of this call option would prevent the system from ever becoming overleveraged. The parking system allows Nubits holders to evade the call option. If a significant fraction of Nubits are parked, nushares holders could lose control of the debt to equity ratio in the system. In the face of an adverse price shock, this may lead to insolvency and a collapse of the peg.