A game like this would be perfect for an elevator battle.
Say the marines are in an elevator and it keeps stopping on every floor for 1D3+2 turns. then the door closes and it proceeds to the next floor and so on and so on until it reaches it's destination.
Note: For those that do not know, a D3 is just a D6 divided by 2 (1-2=1 3-4=2 5-6=3)
To represent the elevator use one of the biggest tiles, The Elevator Door cannot be closed or opened by either side and cannot be destroyed.
The Elevator stays on a floor for 1D3+3 turns then it closes and moves onto the next floor. It takes 1 full turn for the elevator moves between floors.
Example; The First roll is a 2 so the door will be open for 4 (1+3=4) turns.
Turn 1: The marines all go on overwatch/guard or what have you
Turn 2: Stealers charge the marines and the battle is joined
Turn 3: The marines continue to hold out against the onslaught
Turn 4: The marines manage to survive until turn four and the doors close in the mission status phase.
Turn 5: The marines shift position or twiddle their thumbs as the elevator rumbles to the next floor. Any remaining stealers on the previous floor take to vents or run off into the hulk in search of prey
Turn 6: The doors open and a 6 is rolled so the marines have to hold out for 6 (3+3) turns, Hope and prey brothers, the emperor protects.
Now I need to decide if the hallways that the elevator opens onto are randomly generated in some way, or decided already and what a good shape would be.
Door's closing! I already think that this is the one scenario where a door can close on a model. When the turns the door will remain open run out and the door closes any models in the way are crushed with no save. Marines could potentially use CPs to step forward (trapping himself on the floor to make a valiant last stand) or backward (into the relativel safety of the elevator assuming there is room) out of the way of the closing door.
Squad size needs to be determined, Genestealers get 2 blips of reinforcement and 2 starting blips.
Thanks Wilsongrahams for basically compressing everything I meant to say into a small paragraph.
I'll add more as I think of it.