I imagine a tool/service that exposes a single, continuous storage by aggregating already available storage facilities (own VPS, FTP, SFTP, WebDAV, IMAP, cloud storages, …). That means, the aggregated storage hides where the data is actually stored and files can span multiple facilities. It would enable a more efficient use of existing resources.
To achieve redundancy (providers, locations, technology, etc.), a configurable erasure coding would be a plus.
In case confidential data is to be stored, it is inevitable to have strong encryption, so that operators of the storage facilities can not spy on the data.
Luckily, there is an awesome project that almost fulfills the criteria above: Tahoe-LAFS. The only thing left to do is to implement more storage backends…