Two features I would love to have, at least as something moddable are:
1. allow to edit by softcode helm critical protection rules:
in vanilla BG, any character wearing something in the head slot (no matter if it's an helm, a ioun stone or jansen googles) is immune to critical hits. That is imho a bad rule, not just due to its inconsistency with p&p ad&d, but primarily because it removes a feature which makes the game more exciting, moreover sterilizing player skills that rely on critical hits (like some HLA and weapon skills).
This is, sadly, an hardcoded feature, that only by altering the .exe file can be modified. TobEx did just that, allowing modders to remove the crit immunity from headgear.
A third option would however be, IMHO, the most desirable: having headgear giving percentage damage resistance on critical hits only (not on normal hits), so that an helm, despite not giving critical immunity, can at least reduce the lethality of such a burst damage. In order to do this, modders would have to be allowed to softcode such an option on every headgear (being able to decide wether to put or not a % damage reduction that spans from 1% to 50% - the latter making you immune to crits since it halves the damage just dubled by the crit - on every different headslot item).
2. allow to edit by softcode a damage threshold feature which would work as a buffer that prevents a character to be damaged by blows which don't overcome a certain amount of prefixed damage.
In vanilla BG is present a percentage damage reduction which reduces by % the amount of damage received by specific sources of damage like slashing, blunt, missile, magic and such. Albeith useful, this feature is only effective to reduce the lethality of high burst damage, since, on low amounts of damage, even an high % resistance wouldn't matter much, with the clearest example being that a weapon damage of 1 wouldn't be reduced even with 99% damage resistance (the character would be damaged by 1 point of damage, no matter what), whilst the same damage resistance would render the blow of a dragon of 100 dmg to a measly 1 dmg.
Messing too much with damage % canthus make a dragon as strong as a gibberling, but won't weaken a gibberling that much.
The introduction of a damage threshold (like the one present in IWD2 and known, in d&d 3e, as "damage reduction) would instaed allow such a thing, soaking a certain amount of damage and only damaging the character if the base damage is higher. Let's immagine, for example, that a character is protected by a chaing armor that gives him a threshold of 4 against slashing damage: only blows overcoming that amount would damage him; therefore, if hit by a normal short sword (wielded by someone without str bonusses to damage), such a char would suffer hp loss only with a weapon damage roll of 5 or 6 (being damage rolls from 1 to 4 soaked by the armor).
This would matter much especially in BG1, where - like in the mod "full plate and packing steel" is seen - a % damage reduction doesn't do the trick against kobolds, gnolls and the like, being much more important not being hit, and thus having high AC a priority.
Not only such a feature would be interesting in order to make many new spells and abilities similar to those present in D&d3E-IWD2, but it would allow a total revamp of the combat system which is, in my opinion, too much AC centric, especially in Ad&d.
By having a damage threshold system it would be possible to have two different layers of defense against phisical attacks, instead of the sole now represented by AC (being percentage damage resistance only able to reduce damage taken, but not to prevent it): not only, in order to hurt a character, the attacker would have to hit it with the Thac0/ac System. This mechanism would still be applied to check if the defender is "hit", however, to check if such a character is actually damaged, the weapon damage would have to overcome the damage threshold given by the artifical or natural armor of the defender.
It's kinda senseless to me that a ogre as an higher AC than an agile kobold.. an ogre should have a thicker hide, thus making him ignore weak blows, but should be easier to hit.
Ad&d explains this by theorizing AC not as a mere "dodge factor", but, instead, as a mix of "dodge and protection", and that's why a low dex guy in plate armor can have more AC than an agile rogue: ad&d pretends that if Thac0 doesn't meet AC the blow can have either missed OR have been soaked by armor.
This, however, is imho a bad simplification, since it prevents to have a deeper combat system where the "chance to hit" is distinguished by the "chance to damage", thus preventing distinctions between "agile combatants" and "resilient ones".
With a damage threshold the game would become less Thac0 centric.. we could have rogues with Thac0 as high as warriors without unbalancing the game since the firsts could still have difficulties to actually damage what they hit, while warriors and clerics (the latter eventually having a lower Thac0 than rogues/warriors) would benefit, when wearing heavier armor, from a damage reduction that would allow them to soak most of the damage received as well as by the usage of high damage weapons that could penetrate more easily against armor
This would add so much tactical deepness to the game, knowing that you no longer have only Thac0 to consider, but also the damage threshold factor, which, by intensive modding, would be even able to transform itself in a weapon penetration system, where weapon with same damage could differ due to their chance to actually hurt someone wearing armor (something better than the current AC differences of armors against the various damage types).
That's it.. sorry if I was to verbose, but I tried my best to make myself clear to everyone.
I really hope that BGEE programmers will allow me to softcode those features!