I think the Gennadich Team began work on this project by first deciding what aircraft types they wanted to model for the time frame (initially 1917-18), and therefore their planning first involved organizing the plane list, and now the work has progressed into campaign and terrain building (my thinking here). The benefit that this build approach has for us customers, is that we'll likely see 30+ aircraft types available when the game is released!
A sim can also be designed by first considering the air campaigns that might be covered, which would then dictate the terrain and planes needed (the battle area and units involved in the fighting that should be modeled).
JFM, I'm also hoping that GT's designers have decided to put their energy and resources towards building in-depth terrain maps that will directly support the sim's included campaigns and missions, instead of trying to build a broad map of the Western Front. This approach might help keep the map building targeted and 'compartmentalized', and allow for more time to be put towards greater object density and the historical accuracy of the terrain being included.