The Imperial Edition includes:

"A unique development documentary video
An extra campaign map that has dragons fight for control of the Earth
A golden skin for your Dragon Emperor
A beautifully remastered version of the soundtrack by Kirill Pokrovsky
Dragon Commander’s best kept secret: an overview of all the choices and consequences in the game (you’ll be amazed)"
some concept art and early screenshots

The Earth map (over a hundred Rivellonian named countries) is about the largest campaign map.