all i know most parachute operation in serious warfare ended in a big mess. maybe in a the future era you could have a drop-ship that's like the transport ship but flies i.e. can move over water and land (no fuel, no range... just normal movement; however it should ignore all road bonuses). for the modern times one could add a transport helicopter like the Bell UH-1 D. same as drop-ship but could transport only a single infantry unit and needs to be over land at the end of a turn.
for WWII times i could suggest following: remove paratrooper unit. change bombers so they can carry one infantry unit that need a new parachute promotion. furthermore they get a new mission: para drop. so all paratroops aren't any bit underpowered since they can be any infantry type (hmm... heard about the 1st para-spearmen regiment?). additionally you'd get another way of unit transport. guess bombers would need a max rebase range.
EDIT: thinking of it the bomber change would make it reasonable to rework all air transport of units into this system with more advanced plane types able to transport heavier units. then again the AI couldn't understand such air transport stuff so just better to leave it out.