I agree whole-heartedly. The ability to generate skins EASILY is a pretty important piece of the immersion factor. What needs to be done is to remove the skinning process from the terrain. At the moment it is a major effort to skin, or re-skin aircraft, from what a terrain developer does with the terrain and the aircraft. American aircraft that were used by the Brits or the Russians are stuck with American colour schemes and decals.
Also, as I'm on a roll, more early war aircraft for improved historical game play. Right now only the Brits and Germans have a decent early war plane set. Why were the American Volunteer Group so successful against the Japanese? Because the Japanese were flying the A5M Claude fighter, for the most part. Not too many zekes in the air in those days. At present, we can't model these fights accurately because we don't have the aircraft.
If this is troublesome, include the ability for second-party aircraft development. There are LOTS of highly skilled people who would do this (look at the effort that still goes into Red Baron III).
