This is a very simple one.
How do you know if your tap is still running when you leave the bathroom? Worse still, what if a pipe is leaking behind a wall?
How about a simple device with a flow meter and a timer, so that if you let a tap run for more than, for example, 30 minutes, it sounds an alarm? With a silence button, it would be a useful warning that otherwise you are going to wake up with a cold shower, as the tank is empty.