I feel like sharing a heretical idea for a mod.
BG rules are D&D, of course - and random values are presented as dices - optimal for table D&D.
Although we are all used to them, I suppose most BG players remember being startled by mysterious "THAC0" etc.
If all "dice" values would be converted to %, all variables will be on one scale and easier to consider.
i.e. THAC0 11 = -(THAC0 - 20)x5% = 45% chance to hit in attack roll.
Save throw vs. breath 14 = -(Save throw - 20)x5% = 30% chance of save
No effect of actual gameplay, but much easier to imagine...
All nice with dices
d20: 1 point = 5%
d10: 10%
d8: 12,5%
d4: 25%
Slightly worse with
d6: 16,66%
d12: 8,33%
So the list of propositions:
1. Dice roll values convert to % and replace in all menus (Record, items & spells description)
2. Damage rolls shown without dices: 2d12+9 show as 11-33
3. THAC0 replace with THAC10 in % (call the value Attack for example - ATK attribute or else), so THAC0 7 = ATK 115%
4. AC attribute replace with something like Defense DFS (or else), -(AC) + 10, so that AC -1 = DFS 11
And how pleasant it will look at last! Unarmored character has DFS 0, one in full plate + 1 has DFS 11
Negative sides of my proposition 
1. Breaks tradition
2. Intuitively unscrutable game attributes may have contributed to BG popularity. It intrigued, made people rummaging manuals to resolve those mysteries of THAC0, chess game abbreviations 2d6, and "the lower AC, the better"
3. Intuively understandable game variables doesn't force to make arithmetical calculation in mind. Less training - less intellect ^-)
4. This would have been very popular mod 13-14 years ago. Now any person who plays BG know these things by heart and can calculate in if wake up in middle of the night
5. Don't know if it technically possible, and if possible - how to do it... And then, manual edit of all itmes/spells descriptions needed. Somewhat inspiring.
doesn't stretch as far as changing game rules - only presentation of values in other scale, i.e. after engine calculates value (e.g. THAC0 of a character), before it is shown in the interface, additional line converts it by formula "-(AC) + 10", so the highest possible AC looks on screen like 30, the lowest AC looks like 0 (I set AC 10 in my scale as 0 - representing unarmored character; by some effects it may be negative: if AC is 20 it looks -10). This way each AC unit (may be renamed DFS) gives +5% chance to evade hit, and the more DFS player has, the better. Lether armor gives DFS 2, studded 3 etc.
, to present game mathematics conveniently for computer user - hiding dice calculation from user view. Per cent are much easier for understanding of chance/attribute. Player needs to understand dice calculation only if he's going to roll dice by hand on the table.
