Ok, after ditching two maps because they were bad ideas (really bad ideas). I think i finally have a winner!
I had the idea when i was watching miyuzaki's "castle in the sky" (great movie by the way). the story is: The combine have discovered how to make gigantic airships that can be armored like tanks and have a very large amount of firepower. They are taking the the airship 'collosus' through its final test phase, if it fails here the progect will be abandoned. Your mission is to sneak abored, blow shit up, and then get off the thing before it hits the ground.
Sounds rather ambitious, but i have it all planned out and I think i can do it. Wish me luck!