Ground detail, little men in Khaki, horizon bleu and feldgrau would be nice, especially if you can fly the German Schusta/Schlasta ground attack missions.
I would also like to see the forts at Verdun, a few distinguishable airports (e.g. RFC at Balleul(sp) was on the grounds of an asylum, Jasta 5's home at Boistrancourt had distinguished groupl of hangers and was next to a sugarbeet processing plant.), some cathedrals and other landmarks. If they get ahold of period maps, they have the outlines of the forests, roads, fields.
However, I would take a better FM over ground detail as most of the aircombat, particulary in the latter half of the war took place between 5,000 and 10,000 feet. This is where the fight was, going after the recon and spotting aircraft.