Fortress Europe would be a good "broad based" product. You have all those mid-late war warplanes that everyone loves to fly. (109, Spit, Tempest, P-47, P-51, B-17, FW-190, Me-262, etc. The biggest problem is that what makes it such a draw (variety of aircraft over a huge map) also makes it incredibly labour intensive to complete. Dozens of new aircraft, tons of mapping, creating ground objects to bomb, a huge variety of armaments (bombs, rockets, MG's and Cannons) and a host of other issues.

If you can get a P-51 and a B-17, you're going to draw a lot of US players. If you can get the late war British and German aircraft you'll draw a lot of European players. The only problem might be lower sales in Russia (which is a big market for flight sims) since there wouldn't be any Russian content.

There really is no scenario that's all inclusive until you get into very late 44 and 45 where Allied and Soviet aircraft were over the Reich, but who want to play a game where the enemy is being outnumbered (in some cases 10-1) by the allies?