I like Office_Fox's and Kestrel's ideas.
I think that airplanes should refuel and get more turns at each city containing an airport or at external airports. That would give us some real reason to buld them (besides the production and food bonus
)
Bombing infrastructure is also a crucial thing, which is basic for blitz war. The problem would be that if you destroy railroads, you can't blitz in with your tanks.... so you should have a choise with advanced airplaes what improvement to destroy.