Dear Balor,
Most of my suggestions were done by others, so i have not so much to say. The only thing that still strange for me is the research system, in particular in aeronautics (the one I know the more). It is only a little remark and it is not a big problem in the game.
A plane is a combination of airframe/motor/weapons and must be researched as a set. Hard to imagine a jumo turboreactor on a wooden l-153 or a Me-262 with one 7.62 mm machinegun

. But in the game it is totaly possible. It is true that motors and turboreactors must follow specific R&D, but the airframe/weaponry/fuel tank (that in fact is a part of airframe) must be a single R&D and moreover an airframe design can support only one type of motors (with little variations).
In brief it is impossible to upgrade a l-19 squad to a yak squad without BOTH the good airframe an the good motor! For example the Me-262 stays at blueprint and prototype state during years until a correct turboreactor has been build. I just want to say that air squad can only upgrade to higher level only if the airframe level match with the motor level.
A compensation for the combined research airframe/weapon/fuel tanks on a plane can be a higher cost that one in leadership.
It is also important to note that the a turboreactor based airframe is very different that a propeller based airframe, so perhaps it can be more accuary to could lauch research in turboreactor based airframe when the therorical jet engine has been researched.
An another thing that bother me is the cumulative effect of turboreactor with standard propelled planes... Just a few (and extremly bad) prototypes has been build with both propulsion mode.
An another time it is just a little suggestion that do not change the game so much, and i know that a videogame do not need to be logical or historic, it is just a videogame.
Continue your good job on this great game all team!
PS: sorry for my bad english I am not so good with this language.