Feature: Some form of background monitoring of accounts that have not synced for a given length of time, with email notification to either admin or the end user (or both).
Reason: To be informed of remote systems or users that have stopped sending files (such as a desktop or mobile device where the app has crashed).
Contemporaries: Crash Plan (now defunct?) and BackBlaze have this feature.
Example: As the IT Guy within the family I’ll be the first and last line of defence for them losing files, so knowing when their system has stopped syncing by the server-side identifying inactivity and alerting would be beneficial. Also handy for autonomous / unmonitored / headless systems.