Ideally I'd like a cistern that flushes only as long as the button is pressed. I have installed one that does that already, but every now and then some moron manages to flush in such a way that the valve at the bottom of the cistern leaks slightly, or lifts the lid or the cistern half off the wall. The result is that the rainwater tank empties its 30 cubic metres of water into the toilet, and there's no water for anything until the cistern is fixed and it rains. I suppose some kind of syphon cistern would work except that it would do a full flush every time. Or if there was a part-flush mechanism the morons would manage to wreck it. Elsewhere I have a very old syphon cistern that people manage to flush with some weird action so the water runs continuously.
An idea has just occurred to me - to put a very heavy weight on the valve of the existing cistern.