Txurce
Deity
It was proven in WWII that bombers were not enough to kill cities. Bombers have the ability to turn the buildings themselves into ruins but as was proven in Stalingrad and Berlin among other places, if you have a determined enemy that wants to hold the city it doesn't matter how much you drop bombs on the city.
During operation Cobra, (Normandy break out), Bombers absolutely destroyed the German lines, as well as parts of the US lines. So they were very effective against individual units dug in. When they were accurate enough to get to them.
It was debated all through history if carpet and or fire bombing was simply a terrorist startegy to kill civilians and destroy production. (Production was simply moved to other hidden locations so it didn't really hurt that all that much either).
Aircraft should either kill tanks fairly easily or do nothing. 50/50 chance, (they were either on target or off). Perhaps make them invisible in trees and vulenearable in the open.
Tanks on the other hand were unstoppable unless you had some kind of anti tank capabilities. That is how they should play out in Civ. Infantry and Mech infantry should have to swarm tanks to kill them and should die in droves trying to stop them. Tanks should be very powerful!
IMHO Simply making tanks, (and the anti tank ability) much more powerfull should make them more viable to players.
If you ever saw the film Saving Private Ryan, the scene when the tanks are approaching and the ground is shaking and everyone is looking at one another wide eyed. Tanks are scary things. In Civ no one bothers to even build them. They need to be much more powerful, and an emphasis on Anti Tank boosted. Appropriately enough the tank is finally killed by a fighter as it tries to cross the bridge.
1. Bombers should kill population and buildings if used against cities, not effect the defendability of it.
2. Tanks should be all powerful, but vulnerable to anti tank ability.
3. Anti Tank should be a promotion choice all units of the era can take.
4. Fighters/Bombers should have anti tank abilities but somehow be inaccurate.
5. Modern Aircraft should be much more accurate.
6. Dedicated Anti Tank guns should be powerful vs. Tanks but useless against other types of units.
7. Anti Air Guns should be powerful against planes and bombers but useless against other types of units.
Again you could use promotions to make Anti Air good againsts tanks, (German 88). You could also make promotions to make Them good against infantry, (quad 50's).
You raise a lot of interesting points, some about realism, and others about game play. In RL enough bombers can redline infantry and kill tanks, but do not leave cities defenseless against a redlined scout (who seem to always take the strongest redlined cities). This approach would work in Civ 4, where cities were defended by stacks. Extrapolating to Civ 5, where only one unit can defend the city, the ideal would be for bombers to degrade the city defenses to redline, because augmentable "city defenses" can obviously be reduced to rubble. Bombers should have a harder time redlining the defending unit (if it's infantry). I don't think all of this can be accomplished with our present tools.
Going through your list, I agree that tanks should be more powerful, but disagree that:
* bombers can't degrade city defenses - otherwise the numbers are too high
* AT guns should be a ground-unit promotion, because it's too easily achieved
I also think that AT and AA guns should have some effectiveness vs melee, but strong "vs" bonuses.
Finally, keep in mind that fighters and bombers already have melee/armor promotions.