I've had a few ideas concerning this myself. I'd like to offer some of them and see if any inspire debate.
REFUELER
I think a refueling plane would make a great addition. As it stands, I've edited my game so that some of the more modern planes have ranges in the dozens of squares. This is the best way to portray real life, currently. It would be my hope that a refueling unit would function like so:
I have a plane, call it a KC-135, the American refueling plane of choice. It has a range of 15, and some sort of ability to increase the range of other planes. In other words, a refueling unit would allow other planes to operate within a certain radius of it, even if that square was outside of their normal operational range. An example would be sending out a KC-135 unit 15 squares from it's base, and that allows any fighter to operate within six squares of the KC-135's location, since six squares is its normal operational limit.
I hope I was clear on that...
SPY PLANES
I see these as long range planes with a souped up recon ability. Depending on how advanced the plane is, the range they can see increases. A blimp can see two squares, an early spy plane sees three, and a more modern plane, like a Blackbird, can see four.
The actual numbers aren't important. What is important is that the editor include a toggle where you can set how many squares the unit can see. Assuming that a guy floating in a blimp can see as well as a U-2's camera is asinine, and could be easily fixed.
Whether or not spy planes could see units inside cities, or nuclear weapons, would need to be evaluated so as to not upset game balance, though in general, I'd say "Yes" to both.
To offset these advantages, these planes would be expensive and vulnerable to attack, though I do feel that the chance of intercept for a Blackbird should be low, somewhere between the chance of shooting down a fighter and a stealth fighter. The Blackbird's high speed and altitude make it a difficult kill.
SATELLITES
These units should be invulnerable, or nearly so. Certainly no air unit should be able to intercept them. Their range of vision should be high, but not absolute. They definitely need to be able to see units inside of cities and nuclear weapons. I also think they should be able to see submarines.
As for how they should work, I don't think they should be an air unit, per se. I see them as being like ICBMs. Immobile, global range, but instead of nuking a square, they recon it. Cost should be high, as should the required tech level.
I also think that the satellite shouldn't necessarily be lost if the city that built it is captured or destroyed. The loss of the home city would represent the destruction or capture of the ground control station, true, but the satellite itself would remain in orbit. Maybe have a fifty-fifty chance of the unit relocating to another of your cities. If it does not, the unit would be captured or destroyed, depending on your enemy's tech level.
Perhaps a small wonder, call it NORAD or something, could ensure that your satellites always transfer to another city safely.
AWACS
AWACS stands for Airborne Warning And Control System. You'd all recognize an AWACS plane to see one, they're the ones with the giant radar sets on top of them. I think an AWACS should operate similarly to how I described the REFUELER working. It flies out and establishes a radius around itself, maybe five squares. Any of your air units operating in combat within that radius get a bonus to their Attack and Defense figures.
These units would be expensive and vulnerable themselves to interception. You'd need to assign fighters to protect them. I'd imagine that an AWACS and a REFUELER operating together would make a deadly combation, and would add more strategy to air combat.
AWACS, and I suppose REFUELERS, should come in both land- and sea-based varieties. The sea-based ones would have the same abilities, but much reduced range and would only really be useful far from you own home cities and airbases.
Thanks for reading, and I look forward to reading your thoughts on my ideas.
Matt