I'm guessing this is a non starter, for many reasons!
As an Amazon Prime subscriber, I get unlimited photo storage, so I feel a little begrudged that I have to then pay for photo storage with Lightroom CC. It'd be great if you could choose your cloud service provider in the settings.
A couple of issues off the top of my head:
What would you do if you wanted to switch cloud provider in the settings? Would it need to download all the files and then upload them to a different provider?
You can obviously manage/delete/move the files directly in the cloud, which would play havoc with Lightroom.
I expect there could be issues with managing space.
I do however think it would be a real selling point for the service. Essentially using Lightroom CC as the front end and allowing users to manage the files in the cloud their way. It would also make backing up locally easier.