Given the demand of print media - I'd love to see the ability to change margin dimensions (and, although rather more difficult, allow for mirror margins also) to be included.

Given the Creation Tool is essentially creating HTML pages that are then output to PDF, it must be possible to include Margin dimensions for the overall PAGE elements as well as Padding

Mirror margins might well be a PITA to implement though - it would be possible with elements for odd number pages and elements for even numbers ones..