Well, the rules for Killing Blow usually allow only models with US up to 2, so bigger things would be unaffected by Killing Blow.
Personally, I don't think Killing Blow is a very Empire ability. How about: has re-roll to hit and wound against? Or has +1 to hit, +1 to wound against? Or simply just 'hates spellcasters'
I have been thinking quite a bit about this, and have some questions, as I want to keep the captain as plain and simple as possible, while I also like many of the ideas here.
What if the options were magic items? (i.e. Headband of hunting - 10 points, Captain only this model gains the 'scouting' special rule; Hat of the witch hunter - 25 points, this model gains 'hidden' while not mounted and has Killing Blow (or a replacement) against any model that can cast spells (or bound spells)).
What if the options were like this:
Witch hunter: 10 points, cannot take magic items, has killing blow (or a replacement) against models that can cast spells (see above)
Scout: 10 points, cannot be mounted, nor can he wear full plate (or Armours that are the equivalent of Full plate)
Seneschal: 10 points, may only join a knightly orders unit, he and any unit he joins are immune to panic caused by allied infantry.
Leader of men:10 points All state troops within 6" may re-roll failed psychology checks. He may not be mounted
BSB: 25 points
Vanilla: 0 points, allows a unit he joins to re-roll a psychology test once per battle.
EDIT: Alternatively, we could turn the scouting one into a magic item, the Witch Hunter and Seneschal into unit champion upgrades, and make leader of men the basic, once per battle make a reroll on the Captain. This is actually simpler, I think, than trying to balance these different options. Also, we could change witch hunter to something like: 'this unit has eternal hatred against any model than generates dispel dice.'