Branching or Unenroll based on a reply to an email
I think that this feature would be useful for pretty much anyone that uses Hubspot.
The goal would be that when a contact replies to an email from a workflow that you would have the ability to either branch to other actions based on their replying or to unenroll them from the workflow. (This would be awesome for sequences too, come to think of it).
This would be valuable because it would save time spent from manually removing contacts from workflows (and possibly sequences). You could also use it to tailor interaction based on their activity in a way that is not a link click or form based.
Infusionsoft/Keap allows you to do this in their campaign builder if used in tandem with PlusThis and it's insanely useful.