[brained]

I think giving attention to the FOSS feature set is good.
Backups, however, are already fairly well covered.

1. Use LVM when you setup your server.
2. Stop the zimbra processes
3. make snapshot
4. Start the zimbra processes
5. use rsync to duplicate the contents of the snapshot
6. delete the snapshot

Perfect once a day backup with maybe 60 seconds total downtime.

Per user pricing for mobile support would be a better area to give attention to the FOSS edition