Dreadnought I'd agree with what your saying about flight, it's likely there will be a fighter and bomber for each era. Which I think by looking at tech tree is: Flight, Advanced Flight and Stealth.
I think its also likely there will be only 1 helicopter which is used for attack not transport.
But there has been several mentions of a transport plane that carries 2 units at various places (not sure if firaxis have confirmed it though). I think its unrealistic to not have some type of air transport in the modern era of the game.
So speculating on what air units are in the game:
Flight: WW2 Fighter, WW2 Bomber, Air Transport
Advanced Flight: Jet Fighter, Jet Bomber, Helicopter
Stealth: Stealth Fighter, Stealth Bomber