I was actually toying with the idea of putting together a ww2 tank/ infantry mod using either Crysis or Far Cry 2 games.
I used to do some modding in the past building maps and various objects but I always wanted to do a full conversion mod. Both Crysis and FC2 have awesome capabilities with jungle/woods.
Not to mention everything in the game can be destroyed. So if you drive a truck into a building it does damage to the building.. Same with trees, you can knock them over and so on.
I used to do a lot of 3D work and can do the 3D work and some of the textures, but I know little to nothing on the coding end.
In regards to aircraft, I really don't think the game engine would support realistic flight models. Not to mention I think it would be restricted by a limited altitude cap.
The cool thing about Crysis is it already has modern tanks in it. So pretty much all the basic physics are built right into the game. So it could fairly easily be used for a ww2 tank mod. (not really easy, much much easier than having to code all that in)
I'm not 100% sure if Far Cry 2 has tanks, so the physics might have to be added, but FC2's has better terrain damage capability.