There was a time when Napster, Kazaa, eMule were king. The content industry fought against it, but developers came up with decentralized solutions like DHTs or supernodes.
I was convinced the next step would be friend-of-a-friend sharing. You only share files with your direct friends. But they can pass on these files automatically, so their friends can get the files, too. The friends of your friend don't know that the file is originally coming from you, it is completely transparent. The goal is to not let anybody you don't trust know what you are down- or uploading.
I would piggyback on the social graph of existing networks to get started. I actually had a prototype of the friend discovery code, based on XMPP (using custom stanzas). You login with your credentials, and it shows you all your friends that are using the same app. It worked with GTalk, Facebook Messenger, and Skype (via Skype API). One nice feature was that this worked without getting an API key from Facebook etc., or having a central server. I was so pissed when they all changed their APIs to make this impossible. It felt like a conspiracy to stop this use case.
I still think if somebody pulled this off, it might work, and it would be pretty distruptive (in the good and the bad sense of the word). It would be the last word in the debate between "data wants to be free, change society to enable what technology promises" and "data has to be a commodity, restrict technology because in capitalism we can't feed artists otherwise".