Excuse me but... what's the point? Syncthing basically does what you describe (let several device "subscribe to each other" and receive broadcasts that files are new/changed), except without any pesky centralized service, but in an almost fully peer-to-peer fashion (there is a central "discovery" service, I'd rather a DHT but they don't have that for now).
It is, of course, open source and everything, so I'm not sure why I would reinvent the wheel.

The thing above and beyond this that I would like to eventually see is being able to connect to Sia (without any centralized service) and access my files by simply using a passphrase, which could generate some kind of key on the fly.