I thought of this in connection with The Saboteur, so let me just copy my post from those forums:
One of the influences for Sean Devlin is Indiana Jones, right? So how about some combat on and around vehicles, rather than in them.
In the first Indiana Jones movie, there's a scene where Indiana stole a German truck and Nazis are chasing him. Some Nazis are in the truck he stole, and they start to climb around the side of the truck and on top of it. Jones shakes some off, brushes some up against environmental objects, and fights someone battling for control of the vehicle inside the cabin.
In The Last Crusade, Jones is the guy on the outside. He rides up to a tank on a horse and jumps on the tank. At one point, he's squirming around as the driver is trying to crush him against the environment. He also makes it onto the top of the tank, where he battles many soldiers while the tank keeps moving. Earlier in the movie, Jones jumped from one speeding boat to another to battle in that.
John McClane, another influence for Devlin, did this as well. He fought on a plane wing while the plane was moving in Die Hard 2.
If it's not too late to add, this would be fun in The Saboteur. Perhaps only as one level, in which the game controls the vehicles while the player moves around and between them.
If, at least, enemies were able to jump on the side or back of a vehicle, then that would add a new layer of fun as players try to shake or squish the enemies. If the player could jump on a vehicle, then the enemies could try the same thing.
Anyway, the point is: if you're going to have vehicles in your game, consider that vehicles don't have to be objects in a playground -- they can be playgrounds themselves.