It would be cool if you can force events in hammer. For example, a trigger to stop spawning hordes, another one to force spawn of a zombie boss (witch, that hulk thing, etc), and a lot of custom stuff. So you can create really scary events. I'm not scared if I see tons of zombies attacking me, but beeing on a random silence zone, and a witch appearing from the nowhere followed by a scary sound would make me piss in the pants