I'd love to see a nomadic tribe of ghouls in the subway system that got lost a hundred years ago, but when they finally got out the sun was too bright for them and there eyes didn't adjust so they had to stay in, wandering from tunnel to tunnel endlessly, never in the same place twice. Or maybe they're blind like those cave fish because they've been in there so long.
Yeah, also be great to set up shop down their, sneaking away at night to prey on the citizens of Vegas. and dragging them back to your lair to become your next meal. Also be nice to have plenty of feral/regular ghouls.
A giant maze, with no wait out and out of the darkness you hear a rasping cry! be great.