With Avernum: Escape from the Pit being the combined result of more than 15 years of development of different titles, and an engine already tried and tested in a few other games, there is little I can think of to improve the experience. But the one thing I would like to see is more transparency where the mechanisms are concerned.
I'd probably be satisfied with a small addition to either the inventory or statistics screen showing the character's current chance to hit and adjusted damage (after taking into account attributes, skills and perks, curses and blessings, weapon specifics, circumstances, etc), together with similarly reflected statistics in equipment tooltips. This would help derive the exact effects of circumstances such as dual wielding and the related skill, spells, sickness and other character states, and other possible more subtle aspects of the combat mechanics.
Throw in some more stats in the same place, such as evasion rate, armor, chance for extra action points, possibilities to act earlier in combat, lock pick chances, monster evasion chances, lore (these last three depending on how transparent they are supposed to be, since parts of these activities are hidden. These might be more fun to keep obfuscated from the player), etc, all stuff that can be derived from reading the skills but in this suggestion consolidated in one place, and I canna think of anything more to improve my sojourn down into the bowels of a place that is looking better every time I revisit it...
(Normally, I enjoy the sense of mystery and immersion found in not knowing all mechanics, having hidden mechanics as a part of the challenge of particular aspects of or activities in the game, or finding them out on my own, but for all its dramatic and immersive qualities, and the narrative, developing characters and carefully planning their tactics quickly becomes a part of the fun of Avernum, and improved transparency and information on character and combat mechanics often makes this process more satisfying, at least short-term and in the beginning.)