well, there's also repair your gear as it gets messed up
and sleep when you're tired
i think the proper direction to take things would be to find ways of making you feel more than a bunch of armor stats and a pair of hands with a gun.
-looking down and seeing your body
-having masks limit your vision like some mods do.
-being able to mix and match your attire more, and npcs being more varied for the same reason.
-actually animate your character using medkits, eating food, taking medicines, drinking vodka, applying bandages in first person.
-have radiation have more pronounced affects other than just losing health. like coughing blood, lowered stamina, and such where you really need to see a doctor, not just spam rad hypos or vodka till you're better
-be able to recruit fellow stalker bros to go on missions with you if you want, so you're aren't the only dumb schmuck out there lonering it up
-more interaction with your environment. like being able to sit down by campfires and chill, play guitar, eat, drink, smoke and such.
-anomalies and artifacts more mysterious. their affects more unpredictable and weird. Obtaining them should be much more dangerous
-areas of interest farther apart with more anomalies (that can change position) in between so there is more wandering and you can't just sprint back to base for safety whenever shit goes sour.
-mutants that can climb, so you're not safe standing on that rock, or truck