The Adversary doesn't normally sit back and create his own territory; his creations reside within everyone else's. I've got a few ideas, but 'cause it involves everyone's creations it'd be cool to get your opinions.
The first is for the Adversary to create demons that live throughout the world, inside the deep woods, black caves, in the long shadows of crags and mountains and so on. Another is that the Adversary could create several points of occultism within the world -- places where summoning and performing rituals will be much easier. Things like standing stones and pillars in the desert, groves in the woods, columns in the sea; areas of the world that are in tune with the material.
What do you think? D'you have any ideas of your own -- they say two heads are better than one.