I haven't gotten too far into Prey yet so I can't personally speak to how difficult it is, but if I know one thing about PC gamers it's that they love making things harder for themselves. Well, don't look now but that coffee cup on your desk just turned into a hardcore difficulty mod. It's called Prey For Death, created by modder jvames, and it will make your time in Prey a grueling (yet hopefully enjoyable) nightmare.
The mod makes a number of changes, beginning by restoring a few traumas originally cut from the game that now occur when you take damage, like bleeding, burns, fractures, concussions, and partial blindness. These traumas "can be cured with a specific medical cure that you can collect and/or fabricate." The mod also activates a dormant Field Surgeon skill that when acquired will allow you to cure traumas with regular medical kits. Food is no longer as much of a healing benefit as it was, and water doesn't heal Yu (ha ha) at all.
Typhon, your alien enemies, are now both harder to detect and take down. Not only have they received a 20% heath buff, but most music cues that signal enemies have been removed, along with on-screen indicators pointing them out. You'll have to step much more carefully and listen closely to environmental audio to know when enemies are around. Also: "You no longer automatically obtain the mimic detection chip so mimics are more of an ever present and unnerving threat."
Items no longer glow, adding to immersion and forcing you to scavenge much more carefully, and there are now more limits on inventory item stacking. Typhons will no longer drop ammo or grenades when they die, which makes sense—why would they be lugging that stuff around? Ammo is more scarce in general, as much of the loot found on human bodies has been replaced by fabrication licences, making crafting even more essential to survival.
If you're just looking for more immersion without a completely punishing difficulty change, jvames has you covered. The Prey Immersion Mod mostly makes changes to what you see on your HUD, removing the crouch icon, flashlight icon, enemy markers, and stamina and jetpack meters. It also removes item glow and the music cues that accompany the appearance of enemies.