Remember those times you played the narrative scenarios. Always the marines trying to escape from the invader's clutches. Well... how about marines trying to escape from the clutches of other marines? Normal scenario, except the 'invaders' instead use other marines. The deck serves as a timer, and the 'invader' marines can spawn with one of the 'tier 1' weapons.
If a 'marine' marine reaches halfway, the 'invader' marines can respawn with 'tier 2' weapons (sorry, plasma isn't allowed).
If the timer runs out, mark out where the furthest point the 'marine' marines managed to get to. Then switch sides and try to stop the other team from going further. If the 'marine' marines reached the finish, note how many cards are left in the invader deck. Switch sides and try to stop the opponents from finishing faster.
This is just bare ideas, and there's a lot of space for improvements, so post away for ideas.