A few years ago I visited Hong Kong, there they have automatic flushes
that detect a person sitting on the seat, then flushes when that person
gets up. Big snag

Yes. I hate those, usually found in recently refurbished airports. They
don't give you time to close the lid before the flush and they usually
manage to flush about 2 or 3 times during each usage, wasting huge amounts
of water.

The best toilet innovation I've seen is in the US (although it only partly
made up for the cubicle doors, which aren't far off having CCTV of your
movements displayed on the terminal screens). It consisted of an automatic
paper feeder that dragged a fresh bit of paper onto the seat when you
pressed a button, so you don't need to wipe the seat and delicately balance
a couple of sheets on the seat.

Christian.