I am of the opinion, that due to the importance of aviation in later stages of the game, we need more detailed simulation of the aviation component of the game.
I would suggest to:
Add cost of introduction of new aircraft into production - in majority of instances, the customer (air force) was paying at least part of the cost of development of new aircraft. On top of that, the Air Force would have to cover the cost of introduction into production, which is very significant investment. Currently, the player is not constrained in any way in designing and introducing one new aircraft after another.
Actual introduction of new aircraft to front line units should be more gradual - currently many (all?) squadrons get the new type at once.
Combat losses are too cheap - cost of sending night sortie or facing enemy fighters without escort is quite negligible. Replacement rate should be prolonged and the quality impact of losses should be felt more - substantial loss should take more than a month to replace (which is not necessarily realistic, but it is needed due to the monthly turn cycle)
I would suggest to introduce one month stand down period for air units which have been modified (transferred to another base / ship, new aircraft type, new role, ...). Again, this is not realistic as such, but in game the only time segment we have available is one month. It does not take necessarily a month to transfer air squadron from a base to a ship, but it takes some time.