I started toying with using BTSync as a messaging system, where users create outboxes for each other and exchange read-only secrets. The setup is a little cumbersome, and an API that allows for creating and managing shares would help automate nearly all the steps. (edit: Actually, even just a command-line version on non-Linux computers would solve this.) Also, not API-specific, but a write-only share would be interesting, to use as a public inbox for the above usecase, and also more generally. This kind of thing should have the ability to whitelist files based on parameters, like size, extension (regex pattern for the name, perhaps). Being able to customize the write-only secret, like the master secrets can be, would also be great, so they could be made more friendly and suitable for a public "address" of sorts.