The type of derivatives that you are describing are called futures contracts. In fact, HBD is a derivative of sorts.

A derivative is a financial security with a value that is reliant upon or derived from, an underlying asset or group of assets—a benchmark...from Investopedia

When you buy HBD you are guaranteed to receive one US dollar's worth of Hive from the blockchain 3.5 days after you initiate a conversion as long as the debt ratio of HBD against the virtual supply is equal to or less than 10%.

The conversion is done at the price feed at the moment it finalizes and since the later is determined by calculating the median of the last 3.5 days it always lags the market. That lag can either be a benefit or a detriment depending on what direction the price of Hive is moving.

I am not sure what improvement(s) your proposal introduces to the current economics of HBD.