I would really like to see an email volume report that would include all the contacts in my Marketo database and the number of emails they've received every month and if they've unsubscribed. Ideally, I'd like to be able to show that the volume of emails an end user receives directly impacts unsubscribe rates with data.
From what I've read in Community, a report like this doesn't seem to exist in Analytics but I think it would be very beneficial to all Marketo users (and fairly easy to create on the backend)!