maybe one set in the canals were you fight through headcrab zombies to escape and maybe combine could rappel in or use a dropship to get in to add more of a challenge
one set in the city were you control a small group of rebels to escape city 17 but are blocked by combine
another idea i had was one set in a prison (not Nova Prospekt) and your trying to escape like your already armed with something and already broke out of you cell but you fight out of the prison and maybe a button in each cell block could release more rebels

