So it will have to be 1918 July-October, but where?
Therein lies the reason I just look for interesting terrain... and even that I have trouble deciding upon!
I like crazyeddie's Somme 1918 battlefield placements a lot, but the terrain there is fairly flat. Not that it really matters, but I'm partial to the rolling hills of Reims.
There are other areas of the Front that don't change from 1917 to 1918. Basically anything East of Tahure and North of Arras is fairly constant.
Some links that help the historic approach:
http://patriot.net/~townsend/WW1AirMap/
http://patriot.net/~townsend/WW1AirMap2/