The trailer doesn't need to show the Gallia ships, just shadows in the sun kind of effect, building suspense. End it with epic orchastral music and we have a winner trailer. BAIT the audience, MAKE them want to download the pack. DON'T give the whole game away.
Look at the Halo 3: Recon. I want it. Even the Starcraft II trailer made me sick as a parrot that I couldn't play it... But I WANTED IT!