I have always wanted to install a set of speakers in a public space, that instead of emitting sound cancel it out. It would be perfect for one of those very busy and noisy streets in London such as Charring Cross Rd or Oxford Street. They would of course have to be installed in such a way that they are invisible.
This would mean that you might end up walking down the road having a conversation with a friend and suddenly find yourself unable to hear what is being said. It would be like a little bubble of existence right in the middle of the city, where you can't hear any squealing cars or diesel engines of double decker buses. An oasis of complete silence, a place of blissful peace or contemplation? Or maybe a place where you can be as rude to people as you have ever wanted without having to suffer from the consequences?
Just a thought.