This idea certainly is very useful, especially for those Journeys that send many many emails. We'll consider adding this functionality to the Journeys roadmap if there's enough interest from others.
However, sending the emails "in order" is a very narrow use case. Most of our other client's journeys have multiple branches making "in order" meaningless. Perhaps a single "button" (using that term loosely) that sends all the emails of the journey to some user-entered set of email addresses?
@Mike Halekakis -- thoughts?
