Sections of a message body should be targetable to specific segments. When the user sets up a message, he or she could target everybody with a basic theme that everybody receives, and then add a section that only customers in a specific region see. This would allow something like this:

1.) Message Header/Logo (everybody sees this)
2.) Subject Line (everybody sees this)
3.) Hello Customer Name (everybody sees this)
4.) Basic Message (everybody sees this)
5.) Coupon for Customers from Georgia (targeted to one segment)
6.) Coupon for Customers from Nevada (targeted to a different segment)
7.) Coupon for Customers from New York (targeted to a different segment)
8.) Default Coupon (for anybody that didn't fit the above segments)
9.) Footer

This would allow users to customize and give tighter personalization and relevance for their messages without having to create a whole new campaign email each time. It would also facilitate A/B testing.