It would be highly beneficial to be able to report on marketing emails by subscription type.
In other words, this would be the capability to group email performance data by its corresponding subscription, and save that report to dashboards.
Right now, there is option to view some data by subscription type by heading to Marketing > Email > Health > subscription. This beign said, these metrics cannot be saved as report, and they aren't customizable.
I'm a HubSpot employee submitting this idea on the behalf of a customer. For reference, these two existing ideas are similar but not exact: