Greetings,
I am a medical student and so I was always curious if DF could be pushed to more realism in this area. First thing I have done is simple tissue rebalance (you can find it here:
http://www.bay12forums.com/smf/index.php?topic=71494.0 ), which I am trying to balance as finely as possible. As it is just a simple raw modification it wouldn't be too difficult to implement in some new release.
One more thing that currently needs to be implemented but cannot be done from raws is how the game handles blood loss. What I suggest:
All creatures will have certain volume of blood (should be cca 7-10% of creature's mass) and lost blood will regenerate slower (volume 1-2 days, perfectly healthy in 3 weeks). Creatures with blood loss will experience hypovolemic shock, which I suggest will have two levels of severity (IRL it has four, but I think it wouldn't make difference in game mechanics if you just have tachycardia and higher diastolic pressure) depending on amount of blood lost:
under 30% nausea, general weakness, dizziness, may fall on ground;
30-40% as above but also permanent stun, may fall unconscious, may drop held items
more than 40% blood lost: falling unconscious and dying.
What also needs to be polished is how tendons/ligaments and nerves are handed. First off, tendons and ligaments should be concentrated around mobile parts of skeletons - afterall ligaments hold skeleton together and tendons hold muscles on bones and the have to cross a joint, or the muscle would be useless. They should be much harder to injure, but once injured they should cause more severe debilitation of wounded limb.
Nerves - there should be MUCH fewer nerves (for example, human arm only has three motoric nerves, the rest are sensitive and it really doesn't matter THAT much if you lose sensitivity from combat perspective - of course it should disable delicate jobs like surgery or crafting) and they should be harder to reach - an injury that would cause you to lose ability to grasp would almost cut your forearm in two. BUT damaged senitive nerves should cause pain and if you manage to hurt (not tear) a motor nerve it should stun the limb for a while (when you hit your funny bone you are hitting one of your motor nerves).
If this could be implemented, combat and injuries will be much more realistic (it can never be a simulation, but trying to get close is never bad thing) and more fun.