Backstory: DropBox is great but what happens when their server crash or they get hacked?! All those precious files are gone.
The Idea: A decentralized P2P file sharing platform.
Deep Dive: Similar to DropBox, you would build a platform for users to upload and store their files. But, this would work over a p2p network where user's files are split across multiple hosts and backed up for redundancy. Similar to bit torrent, users could allocate a portion of their unused hard drive space to give to the network in exchange for additional network storage.
Validation: DropBox is wildly successful, this is a good twist with a value proposition for super users.
Monetization: Like DropBox, you would charge a monthly subscription fee; however, users could reduce their bill by allocating GBs of personal hard drive space to the network.