It seems simple to me - beyond a certain threshhold, the airline should have to make cash payments to the pax. For example, beyond 4 hours, $100/pax/hour. That gives the airlines a strong incentive to return to the gate, but still allows them some flexibility in the case where after 3:45, the plane has clearance to take off in 30 minutes, and returning to the gate might be costlier than the cash penalty.