Pontification follows:
The Fiat CR-42 or 32, HS-129, just because they look great, hanger queens or not. Frankly the planeset is deep enough to satisfy players. I am sure there is a chart somewhere that shows which planes, regardless of when they were added, are used most. Point is, what aircraft are significant enough in a historical sense to add to the set? Or, is it, which plane if added, will aid in player base retention?
I think many might be drawn to curiosities, such as the 50mm 262, fly it once or twice then realize the standard loadout is more devastating and survivable.
IMHO all plane additions need to play into the greater notion of how the game is played. If you want furballs, early war turny planes are the best. If you want some measure of strategy, then the game should be arranged in such a way that a collective strategy works, then attack planes such as the beaufighter make wonderful sense. If you are looking for historical fights for scenarios, is there sufficient interest in scenarios to merit the thousands of hours required to model a new ride? (I wonder if 3D scanners help cut the time... Greebo?)
I think investing time in the terrain and environment is where HTC is wisely spending their time. If they found a way to say create clouds that do not crash systems, places like TT could be GVing and ground attack paradise, while furballers have their way above the clouds. Weather would be nice.