I enjoy the flag feature personally. One thing which could be added would be a mechanic for calling votes on various things. Corresponding to this would be the ability for the leader to either give or remove a member's voting rights.
Quote from oceanhawk: “Transport aircraft and Paratroopers.. I dont think you can have one without the other it would mean that lets say, you manage to go behind enemy lines.. You capture an airfield.. Now you can fly in reinforcements.. ” Transport aircraft in and of themselves would be good. Of course, they couldn't carry tanks or anything but it would be a great benefit to be able to quickly send units from one side of the map to the other.