I agree with the idea of implementing a risk-free rate for FEI. Such a figure is critical to the functionality of many high order derivatives such as options (Black-Scholes pricing model), CAPM etc.
Though instead of voting for a static interest rate, I would like to call to attention to my earlier idea: Idea: Fei Protocol Target Rate
Which proposes an policy interest rate based on a basket of external market indicators, and can be transparently reviewed on a regular basis. The rate would be hovering around 3.8% currently. other proxies scuh as DAI’s internal policy interest rates can be taken into consideration aswell, to adjust the rate downward as required.