Here's a proposal for passive weapon-based battle skills that I think would:
(1) favor combat in the open over tunnel combat,
(2) provide melee combat flavor, and
(3) balance weapon categories and make some (lesser-used) weapons more interesting.
Add some of these, or similar, melee-based skills:
Parry: chance to block an attack, (if not using a shield). If this ability (has a chance to) apply to attacks from multiple monsters, it would benefit open combat.
Riposte: chance of an extra attack after a parry (or a shield block). Probably should be the same as parry, kicking in a higher skill levels.
Disarm: on hit, chance of disarming (forcing weapon drop).
Repel: on hit, chance of pushing smaller monsters back one space. Only works if the monster isn't next to a wall, which would help it to regain its balance.
Double: on hit, a chance of an extra attack (probably with lower damage/acc) on a monster adjacent to the attacked monster.
Remise or Reprise: similar to double, but after a missed or blocked attack. Could be combined with Double.
Reach: much like reaching currently. Higher skill levels let you reach with more acc, more damage, though larger monsters, or into deeper water.
Stab: same as stabbing at present.
(1) All of the skills (except stab) would make it easier to battle multiple monsters in the open, opening up a refreshing alternative to cowering in tunnels.
(2) They would primarily make combat more flavorful ("You parry the nerd's attack but your riposte misses"; "You miss the cow, but your remise hits the moose"; "You hit the smurf, who [stumbles backward] [uses the wall to retains its balance]"). Ranged combat is getting a wonderful flavorful boost in 0.6; it would be nice for melee to get one, too.
(3) The skills granted by each weapon could be based on flavor and balance considerations. This could help to subtly weaken axes, improve M&F and polearms, or whatever balance issues others might see. Each weapon type would either employ/train the skill or not.
Short sword: parry, remise and stabbing.
Sabre: might train remise and disarm.
Maces, most flails and quarterstaves: repel and double (the idea is that the weapon bouncing off one monster into another).
A halberd and most polearms: repel and reach. (It would be fun to repel a monster and the reach to hit it!)
Axes might train only remise.
UC might train repel, remise and stabbing.
The skill level, the weapon type, and the HD of the monster enter into the likelihood of the skill being activated. For example, you'll never parry an attack with a hand axe, regardless of how skilled you are at parrying. (This is the simplest implementation, but it might make for interesting strategy if a longsword-wielder could choose to train up parrying using a short sword so as to parry occasionally with the longsword.)
These should be activated relatively rarely. The likelihood of parrying, while wielding an appropriate weapon type, might be [1+[min(parrying_skill-2*monster_HD),0]]/100 or something in that ballpark.
The proposed thief god, or other gods as appropriate, help train otherwise give a bonus to the skill. Monsters would use these abilities, too, if they wield the appropriate weapon.
They are passive, based on the idea that the player uses the ability when the monster presents the opening to do so, although some (like reach and repel) might be better as "v" abilities. I suggested more than I think should be added in the end. In particular, riposte, remise and double could be combined. Other ideas are very welcome. I'd love to help with a patch but my coding days are years behind me, sorry.