Gunskills would now have much heavier effect on waivering and general accuracy than what it is in F3 or New Vegas, utilizing the skill and STR requirement system from New Vegas (but more heavyhandedly). In addition, the players stance, movement, weapons type and recoil also would affect it.
The normal (according to skill) situation would be standing still and aiming through iron sights (firing from hip would give a large hit on accuracy). Crouching would give a small bonus to accuracy and going prone would give a slightly bigger bonus (with the bonus from aiming coming on top of that). The tradeoff with going prone and being accurate would be extremely slow moving and turning, and it would take its time for the player to get up and ready the weapon again. Firing from the hip would cause bigger spread. Movement would also give a hit to accuracy -- the faster you move, the bigger the waivering with ironsights and spread with hipfiring. Recoil would work dynamically based on the gun used, and would throw the aim off a bit with each shot (while bursting, the amount of recoil per shot would stack up eventually leading to firing straight up -- with hipfire, the "offaim" would be a bit smaller, but the spread would increase).
Guns would do generally more damage and the damagestats would be ranges. IE: Huntingrifle - dmg 11-20, like in Fallout 1&2, but with growth of related skill raising the minimum amount closer to the maximum (though not as far as up to having a static damage, there would always be some range left).
The combat overall would go through a total overhaul. No more run 'n gun FPS bullsh*t but more slowpaced, focused and tactical. A RTwP/TB (optable) setup with full control over the player character and partymembers would be nice. But of course that ain't gonna happen since nothing but horrid FPS twitching is viable way of gaming anymore.
So instead, actionpoints would count in combat.
A simple example:
Lesser cost rate:
Shooting (very low cost) - shooting with not enough AP's left would give a hefty penalty to accuracy and recoil control
Reload (low cost) - reloading with not enough AP's left would triple the time it takes to reload and increase the chances for reload failure
Moderate cost rate:
Using quick keys (cost rate depending on what's being done, changing a weapon (moderate cost), using stims (higher moderate cost), changing ammunition (high moderate cost due to including reloading)) - with not enough AP's, quick keys won't work, no stimspamming or changing weapons, tough luck.
High cost:
Inventory access - with not enough AP's, inventory access is denied, run for cover and wait for the AP's to refill.
Using items inside inventory - if AP's run out during doing something in the inventory, you can still browse and assign quick keys, but after that only action allowed is to exit inventory.
Additionally, no AP's left would increase running speed for... say.... oh well, for examples sake, 20% to provide chances to get to cover while AP's recharge.
And AP's would recharge much slower while moving and at normal rate while being still.
All that only during combat (and during shooting/whacking thin air when not in combat for the lesser cost actions).
Consider AP's in the lesser cost section as representing exhaustion.
And in the moderate and high cost sections as representing mental strength, situtional loss of focus, a panic of sorts leading to indesicion and inability to operate properly.
NPC's would also have their own AP's which would dictate their performance (a bit differently than the PC, since NPC's don't use quick keys or the sort).
In vats you would now have an option to choose a firing mode. Rapid fire - a hastily aimed rapid shooting towards the target; or aimed shots, which would be the opposite of rapid fire.
Rapid fire would lower the accuracy a bit and you could only target a foe as a whole; but it would spend less actionpoints, while aimed shots would cost more and calculate the accuracy without minuses, and you would be able to target specific body parts. The bonuses and hits of chosen stance would be similiar to those in realtime firing.
Being prone in vats would force you to choose a firing sector (so that the player doesn't spin like a dreidel in all directions while being the most accurate he can). Prone position would also be the most expensive stance to fire from, while standing would be the cheapest, and being crouched in the middle. The player would be able to change his preferred stance in VATS, but at a cost.
About craftable weapons... I once toyed around with an idea where there was to be 3 categories of weapons based on their craftmanship. I can't quite recall all the specifics, but the general idea was to have:
Category 1: Post war selfmade weaponry (craftable, high damage output, cheap and easy to repair, high availability, high modability, fairly cheap -- but, non reliable (prone to jam and reload disorders regardless of CND), low accuracy, fast deterioration rate (constant need of repairing)).
Category 2: Pre war Home defense weaponry (more reliable, more accurate, lesser deterioration rate -- but, less damage output, more expensive, less available, moderately harder to repair, non-craftable and less modable).
Category 3: Pre war military grade weaponry (Very high damage output, very high accuracy, very high reliability -- but, extremely expensive and rare, barely modable, non-craftable, very hard to repair).
Something along those lines, and with each category having 1-3 (max) base weapons for each type of weapon (pistols, SMG's, rifles, assault rifles, machineguns, shotguns etc). You'd that way have an entire category of highly modable weapons to craft from scratch and alternatives which have their ups and downs over that category. Weaponcrafting could be handled through related skillthresholds (what ever the skillsetup for that sort of crafting would be).
Close combat penalties for firearms (especially heavy and long arms).
- In face to face combat where the other side is toting a firearm, a chance (manual triggering for the player, but chance based success) for the NPC to perform defensive move and shove the players gun and facing off and slightly stagger the player (and possibly then deliver a more damaging blow/s). This to remove the horrendous way it is now where... you know how it is, and possibly slightly improve closecombat.
Also, damage bonus for melee/HtH enemies (animals included) that come in numbers (overwhelm the player). One guy with a knife or one gecko might seem harmless to your assault rifle toting combatarmor wearing character, but if there are three or more, you better dispatch them before they reach you or you are in serious trouble no matter what armor you wear (if the DT eats all the damage, turn it into fatigue damage that'll eventually collapse the PC after which the foes start to pummel the "weak spots" in the armor and healt starts to drop).
Generally, turn up the damage done by weapons -- the previous situation might seem that it puts a melee oriented PC in a rougher than needed position, but if you are skilled enough, and if the damage values are higher, you may be able to drop one or two before they get to overwhelm you (if you see a group or powerarmored melee units, though, you might want to reconsider attacking them with melee yourself unless you are absolutely skilled and geared enough).
In addition, projectile weapon skills affecting accuracy, not damage (except for - possibly - higher critical dmg multipliers). And locational armor DT values (head, torso, feet, hands).
Enemies and the PC are generally more easily taken down by weapons (no more people eating 30 bullets or 50 stabs and still coming on strong), but only if you know how to use them.
Alernatively - for VATS - it could remain functionally almost as it is, but it would be made a separate phase based combat mode altogether (at players choice) where you would queu up actions and execute them as you do now, but instead of the game hopping back in realtime, it remains in phasebased mode and the enemies would get their actions through the execution phase based on their own set of action points - kinda like how combat in Wizardry 8 and, to a lesser extent, Wasteland works. This would allow for more precise and accurate control of the possible companions too than what we see in the games now.