I had this EXACT same idea.
I don't really like the ship loadout for the same reason as Yami-Yagari stated.
I'll go ahead and add my ideas to this discussion.
First of all, the Leviathan (I like this name better) would have to be allowed a couple deaths, not sure.
The match would be a 1vs3 or a 1vs4, this would determine the amount of people on the Leviathan. To make it easier, so you don't have 12 people crawling over an entire ship, there would be three or four (preferably three (so 1vs3)) "ships" that make up the Leviathan. What I am saying is that there are three sections that make up the ship that don't actually connect in gameplay, but make up the entire ship. There would be the bridge, the boiler room, and the gun deck. Each area would be configured like a normal ship, with a Captain slot and three crewmen slots.
The Captain of the bridge would be actual Captain and obviously pilot (though, on a ship this large and cumbersome, the Captain would not actually need to fly). The bridge would be where you get the most visibility, and the where the wheel for the ship is. I have not figured out what the other crewman aboard the bridge could do other than spot, though.
The gun deck would be, well, the gun deck. All the guns of the ship would be located here and all the crewmen aboard the gun deck would naturally be gunners. They however, would not be able to interact with their guns in order to fix them. The Captain of this deck would be Chief Gunner.
The boiler room would be beneath the gun deck. This area would be made up entirely of engineers, all of the hardpoints for fixing would be located here as well as hardpoints for the guns beneath the guns for the engineers to interact with.
Communication and team work would be a HUGE factor in this game mode. The Captain, Chief Gunner, and Chief Engineer would all be able to communicate to each other via the usual Captain Chat. The three, being so vital to each other, would have to work well together in order to win.
The same goes for the side of the three smaller, normal airships. They would have to have good communication, make good use of terrain in order to get close and into blind spots.
If you guys did create this mode, you could use it as a HUGE event in Adventure mode, with one prosperous city creating a Leviathan and setting out on a glorious mission of conquest, while the other cities have to band together to vanquish the foe!