Gun knockdowns, which makes all guns and most melee will knock enemies, and you, down to the ground, dependent on the target's type and current health. A pistol won't knock down a big bad super mutant ever, but a raider with a couple chunks out of him already will likely go down.
Bleeding, which makes all none-blunt weapons cause varying degrees of health-eating bleeding, which can be stopped by applying bandages, which can be found on various wasteland NPCs, for sale from a doctor or general survival supply vendor, or in med kits.
Hold breath aim, this adds in the handy FPS feature of being able to hold Shift while aiming to steady your gun's sway temporarily. How long you can hold your breath for depends on your endurance.
Rarer guns & ammo, especially automatic/high powered ones. >>> More expensive guns, given how rare they are.
Increased overall damage, and/or increased locational damage (headshots do even MORE damage, limbs even less).
More effective armor, and much more effective power armor, makes good (leather, combat, power) armor last longer and provide better protection, kind of to balance the increased damage.
Static HP, every human NPC's health is about 100 + 5xEndurance, so that some people aren't somehow able to lose 1000x more blood than others, or eat 50 shotgun shells from 2 feet away, even at higher levels, this affect's the player too though. This REALLY makes an enemy's armor an indicator of how much it'll take to take them down, no more saying "well, I hope these 6 assault rifle clips will be enough for that raider".
And that's about it. I've left out things like hunger, thirst, rest, and proper down-the-sights aiming because those are already very well covered by other mods out there. But if you can think of anything else that would add to the game's immersion, realism, or intensity/challengingness (not a word, lol) and that feels realistic and fits with the Fallout world, then post away, anything from combat changes to perks or even (simple) new items (I can do Blender and GECK, and have been fighting with Nifskope since Oblivion was released, I WILL get it to work for me, lol).
PS: all the features I've listed are totally modular, as some of them would make the mod a little too extreme for some, but until I get enough time to make a complete rebuild of the mod, you'll have to create a bashed patch or whatever to get some of the different features to work with each other, as they make changes to the same objects but different areas, but such is the conundrum of mod compatibility.