Currently, the majority of water saving devices in the U.S. allow cold water to escape and shut off when the hot water arrives - preventing the loss of water heat energy, but not water itself. It's incredibly wasteful - especially in areas like California that are water stressed.
Design & sell a device that redirects (or saves) cold water until the hot water arrives. Saved cold water can then be used to cool the hot shower water.
An Australian product called Cullector already does this - perhaps they need a distributor in your region?