Instead of no enemies at all, why not have a few super-powerful, randomly-generated monstrosities (DF forgotten beasts) running around? Make them big, ugly, scary, very hard to kill, and very, very dangerous. Also, more temple-like areas, more traps, more pitfalls. And make the rewards for dealing with these things more, well, rewarding, like the only source of a vital resource (for example, gasoline, batteries, or lighter fluid). Right now, players might screw with nether beasts or the fault because it’s something to do and because they want artifacts which may or may not be a bad idea to pick up, but there’s not a lot of reason to do it.
The thing is, endlessly running from zombies or running them over in cars might be thrilling, but it gets old. We know what we’re going to face and how to deal with it. Removing generic “meh” not-really-challenges-anymore in favor of the genuinely unknown and scary is a good way to keep the empty feeling of a deserted world but still keep the player’s hackles raised.