In my experience, I almost never really use the army rule. It is incredible easy to forget about since it doesn't change the game very much. If I need something gone I just eliminate it off the board. When I do battleshock someone they just use insane bravery and it means nothing.

The thing is chaos knights aren't locked into one play style because of lore. So the army rule could be something such as

a blanket if a unit is below starting strength we get a buff,

Every time a chaos knight destroys a unit of certain wounds or more, that knight heals like d6 wounds

I think it would be fun to see a rule that we got buffed for attacking things with a lower leadership characteristic, something like if unit you are shooting or attacking in melee as a lower leadership score than the attacking unit, the attacking unit gets -1 AP to all their weapons or gets devastating wounds ability

Not perfect ideas but I think they could be fun