I must say that I do like the style of the promo art, however. What is the art direction you have in mind? Because the current art style reminds me of those old 1960s Civil War Trading Cards a la these ( http://www.authentichistory.com/1860-1865/3-1962cards/index.html ) I think unit cards in this style would be awesome and act as a good nod to the retro world.
There are so many strategy games out there that look photorealistic, which is nice, but seeing a historical game wit a more creative design, whilst retaining accuracy would be even better.