I think Smart Sync could be improved by increasing how flexible the move to online-only is. Either:
a) Set a "last modification" threshold so Smart Sync automatically moves files not modified in the last "x" months online-only to free up space
b) Set a maximum Local Dropbox file storage level so that once local storage goes beyond this, the oldest last modified files get moved online only
At present Smart Sync gives no indiciation of what threshold or criteria it uses to move files online only, and allowing flexibility in these settings will allow users to manage their local storage more effectively.