Designing a FET switching circuit that fails open
One of the standard ways in functional safety when a disconnect function is done electronic is to have 2 or more series switches, so in case one fails you still have control and are able to change the system to a safe state. 
So in your case I would add a high side switch to the system so when the you detect that the system has fail, via feedback in your system, you switch off power to motor changing it to the safe state.