What the title said [first post, tell me to shut up now
]:
In the history of the Empire, in my estimation, victory is marked not by conquest or acquisition, but by survival. Unlike Chaos, which must destroy to triumph, the existence of the Empire is enough to satisfy its people. Like Bismarck's Germany, our beloved Empire is a satisfied state, there is nowhere it needs to expand.
Because of this, instruments of assault are out of character for halberdiers. Armor piercing is what I am mainly referring to, but improving WS, etc. are similar. Making them fight better will do two things: first of all, it will only marginally improve their prowess, secondly, it morphs them into something of an offensive unit.
In our ruleset, however, the way units of halberds theoretically win combat is more in line with the defensive nature of the Empire. They take the charge and win not by fighting but by having higher static combat resolution - how many soldiers the enemy can kill is immaterial because our troops are too strong in number and in will.
I feel that the best representation of this would be to reduce what the enemy is capable of killing, or nullifying the effects of casualties. A small ward save, 6+ or at most 5+ would help to represent the touch of Sigmar in the fighting of the Empire. The purpose is not to represent that an individual halberdier is difficult to kill, but rather to represent the somewhat inexhaustible supply of halberdiers - the mainstay troops of the Empire.
Heavy Armor, a suggestion I have read frequently as well, would attempt to increase the defensibility of an individual halberdier. Individual halberdiers, however, aren't great in defense - see the picture on the page before the index of the rulebook, one is getting his face busted. The strength of the Empire is numerical, and numbers are impersonal - like ward saves.
Rules wise, a slightly more effective offensive unit would never be taken over a similarly priced unit of swordsmen with their better save, given the general proliferation of overpowering offense in Warhammer. A choice between a unit with a 5+ wardsave and a unit of swords with a 4+ combat armorsave, however, would be much more difficult. The 5+ save would be more versatile, but by no means impenetrable.
Pointswise, I would suggest that halberdiers with this organic ability should not be more expensive than swordsmen with WS4, I4, and a 4+ combat armor save.
It would not imbalance the game because they would still frankly suck, they just die a little slower.
Cheers if you read any of this