I envision even simpler and more practical thing, under-the-floor heating with addition of disposal water.

Me and my wife are taking shower daily, hers being hotter than mine (apparently that's common difference between female & male).

Every time, I am thinking that so much heat is going down the drain as waste, why not circulate this under the bathroom floor until it cools down.

Since we have a tub, I usually close the drain, sit/stand on a bench above the tub, rinse thoroughly at the end, keeping hot water afloat for next 20 minutes.

Since tub itself absorbs the heat, even after I open the drain and let go all the water, tub itself stays warmy for an hour or so.