The more semi-intelligent you want this to be, the larger the chances it will outsmart the home owner
Seriously, having it look for complicated, multiday patterns in hot water useage will be, well, complicated. How about this: A superinsulated tank that shuts off when noone is home and at night (it could be programmed like the thermostat of a home's heating/AC system), or just before. After that is a small on-demand heater in case too much hot water gets used up and/or is needed at a non-normal time (guests, holidays, etc.).
Of course, semi-inteligent, complicated, multiday pattern recognition is interesting in its own right...