So a huge reason showers, especially long showers, use a lot of water is that the water is only used once before it goes down the drain, but what if we had the option to reuse the shower water for each session? Fill an internal reservoir or the tub with just a few litres of water, and use a pump to recirculate it, with electronic controls for when to drain the water. It could be like a dishwasher: use one round of water for the main washing that actually gets the grime off, and then a little bit more fresh water to rinse completely clean. Something like this could also allow you to take as long a shower as you want or have one of those massaging showers with a rain head and like ten body sprays, and still use less water than with a conventional shower. A panel would control when the water is replaced and when the recirculation is disabled (for example, when using soap so soapy water doesn’t spray back into your eyes). I think something like this would go a long way toward reducing household water use.
Yes, using what we already have is often the greenest option.
I really liked to read about this certain eco brand where they did not use Instagram to advertise their product because the platform itself promotes fast consumption and it was against the whole point of the brand. If you are a truly eco brand you should be also advocating for people NOT to buy your product (sounds counter intuitive!) only if you really need it and after trying repairing what you have or buying used (Patagonia brand tries to come across like this; Fairphone tells you that the greenest phone is the one you already have, etc). This is also why the vision of the future we should have is this scrappy low tech place instead of flying cars and such.
But keep having ideas! The world needs it. I v seen at least a couple of concepts of showers with that system already, and quite fancy, but perhaps you could come up with a low tech one ( have you seen the camping outdoor showers?). Also look at the many solutions of graywater reuse. The ones I really like are home made solutions like someone connecting the shower water to their flusher, or to a pipe that goes to their indoor vegetable garden…