Ok so we limit path finding. Then we are right back to horde cubes. Unpredictability is nearly impossible to make a effective threat with the current Zed skills. Concrete can survive any solo zombie minus the Demo. Now a demo that random blows up would add some fear. But is that fair gameplay? Splitting the group of Zombies by adding idiot zeds just makes the death corridor more effective. Honestly we need some Zombies with skills to manage around. Like if the cop spit actually could penetrate bars and spread. Have the burner have a napalm type move in later GS, A zombie that could rip you from a open space. ( I know very L4D2 ). The party girl shooting Pool Balls ala Zombie Strippers lol. but that is honestly the only way I could see adding challenge without removing traps and such.