I wish there was a way to see all emails and make final edits to /customize them before I hit send.

In Microsoft Word there is a "Finish & Merge" option. The drop-down menu then gives you the option to "Edit individual Documents."

I wish RE NXT had this capability so that I could go back and check that all merge fields pulled correctly.

I would also use this feature to send group emails. For example, a group came to our most recent event. I have the email of the person who registered the group  but was unable to track down the emails of the attendees they registered. In this instance, I wish I could customize that one email to thank the main registrant for bringing a group or address the email to everyone that attended with them. 

If I want to customize anything on an email, as far as I know, I have to go and create a brand new email each time which is very time consuming.