Most of the DLC for getting older Fire Emblem characters reuse the same base map over and over (I've seen about four choices being used), while the optional side-missions seem to be a bit more original. Almost all the old-character DLC uses their official art from their games, which can be a bit jarring, as it really stands out aesthetically, and also makes the units feel really generic and unimportant. But these are pretty small problems compared to how much fun people have been having with the game so far.
According to Iwata asks, this Fire emblem was originally going to be set in a modern setting on Mars. The planet, Mars.Fire Emblem X Advance Wars X Super Robot Wars!
But it has to be Originals only, else it'll never get localized.