This could be nice, but you'd definitely need to shell out for a paid iCloud Drive storage share. The default 5GB won't cut it.
I hope somebody comes up with a way of automatically mirroring files to iCloud drive. It'd be nice, for example, to have specific items in my Documents folder which I tag for automatic backup. Not quite like Time Machine. I think somebody will inevitably figure this out now that Apple officially allows you to dump non-AppStore-related files there.