You've already seen something I'd like to see in the sequel in my review, which basically comes down to: more!
Maybe add a weather system? Weather in the zone could prove to be very mercurial, and punishing.
One of the systems that was removed from the first STALKER video game, and then put back in by modders, was a regular occurring sort of blowout/emission - like a huge storm, which would instantly kill you, and you had a short time to find some cover. Added a bit of panic, to the game.
Maybe have a lethal creature hunting the player, on a bad roll it shows up and has to be fought long enough to be able to escape to a next zone? Now I'm just thinking out loud.. :P