You know what I haven't seen, suprisingly, are the old OFP missions where it just takes advantage of the game features. Like the ones where you just race around the map in different vehicles. I guess that stuff doesn't seem new or exciting anymore for most people. However, a lot of stuff would be great if it were brought to the new ArmA. Some just for fun maps would probably attract a lot of people on the public servers. As it is now, most servers have a tactical map up, and half the people want to mess around and be stupid.
My official idea is to have each player start out in a different section of Chernarus. They will have to use AI teammates to fight past the enemy. It will be such a relief when they meet up with real people. After the rendezvous, you could have them do the main objective. Alternatively, you could make it a race to the center of the map, where the person who gets to the top of the hill has to hold it for 2 minutes accumulatively (King of the Hill). You could probably automagically spawn the person who's currently the King into a tank (The T72, with which you can shoot the fuel drums in the back to blow it up). LOL, hope you like my idea.