If you pre-order the game you get the imperial edition for free.
As far as I know the Dragons on Earth map is just an extra campaign map based on Earth (practically I don't know if the smallest countries could be individually represented, since there needs to be enough room on the map to show units and buildings, as well as gold/research/population).
The custom campaign in single player allows you to play on a world map of your choice, as does a multi-player campaign. If the Dragons on Earth map is anything more than an additional (presumably large) map not set in Rivellon, there has not been anything official to indicate that. It is probably safe to assume there is no additional story content, etc.
