One of the sucky things about mapping in L4D without the official SDK is that Tanks and Witches don't make an appearance automatically, they must be manually injected into the gameplay. While if you're fast enough to turn cheats on, spawn the boss, and then turn cheats off again before anyone can turn themselves invincible, the bosses will be expected thus ruining the suspense and surprise of the event while everyone has their molotovs and auto-shotties out.
Mapping around these events however, even when automated, provides some unique challenges.
The Tank is one mother of a destructive force. He needs a lot of room to maneuver around in, a lot of space to chase the survivors down, and especially a lot of physics objects to throw around.
Setting the stage for the Tank also might require large areas where you can climb to higher ground in, something I'm trying so hard to implement properly. Just hope you don't run into him down a long hallway.
The Witch on the other hand doesn't require as much of a special setup as her underling. All you really need is enough space to avoid her, or enough objects in the way between you and her so that you can sneak in to attempt a single-hitter.
I always absolutely hate it though when she's always right down the path I need to take - I'll make sure there's a lot of that in these maps.