The world has solved this problem with fancy toilets in public places I've seen, flush handles that move in either direction depending on whether it's a good time to be saving water at all. But I haven't seen a shower head with something like that... unless my google skills have failed? Is it out there?
It just needs a nice little button or something you can push to get a satisfying gush of water for sixty seconds, then it automatically goes back to being responsible.
Speaking of inventions... I'd also like to see a stroller that doesn't make you fuss with all those annoying straps to get your baby to set in it. I decided last weekend that Velcro would be good for this. Just provide a huge Velcro onesie for the kiddo, have the inside of the stroller soft and fuzzy, and SCHLIPP... stuck baby!
I am full of wish lists.