how about taking elements of all the suggestions and on certain maps that do not have helicopters/jets, there could be the American troops whose sole permament spawn point is the circling troop carrying aircraft. by capturing ground flags, they establish a ground base. But i would envisage that this means that all American troops can parachute down on to any or all flags, so using the Gulf of Oman idea, the troop plane could circle only on the first line of defense flags, so that in order to capture the others, you must first capture the first line. this would resolve the issue/debate of the game balance.
Its worth pointing out that this idea would also make a point of the airborne ribbon, that seems fairly pointless to me, as in order to earn it i found that i had to actually TRY and obtain it. ie - fly a jet up above 500 feet and jump out, open chute and glide cheerfully down to the ground again.
Paratroopers wouldnt unsettle the game balance though. most of the time a defense will kill a few of them on the way down, but as long as a squad leader survives, spawning on him shouldnt be a problem.
what bf2 hasnt shown us yet is different weather types or indeed darker skies. now a parachute drop in the evening would in some order make the skies more difficult to scan......