Flight is an area that would be high on my list for expanding (if I ever got round to building my own scenarios etc.).
I'd like to see something like:
Advance: Flight, Units: Biplanes (mainly for recon, limited attack, no bombard), Zeppelins (a la Civ2).
Advance: Aerial Combat, Units: WW2 style fighters & bombers, aircraft carriers, airfields, airports (maybe), anti-aircraft guns.
Advance: Jet Flight, Units: Modern Jets, Nuclear Carriers (larger capacity and better defensive strength)
Advance: Stealth, Units: Stealth fighters etc.
Or something like that anyway.