I like the idea that a captain can have a special rule where the unit gets to reroll ld test in case of panic. But I would have it so that the captain replaces the champion and has to stay with the unit. He also has to be the same troop type. Otherwise he can have normal equipment as any captain.
Unit Captain 50 pts.
Replaces champion
Can't leave the unit
Has to be same troop type as unit
Unit can reroll any panic test
So its basicly a normal captain that has been around that particular unit (whichever that is) for a long time and the men trust him.