A. Black cleaver applies a self buff of 30% physical shred, not a 30% shred on the target. Removes group utility.
B. Black cleaver's flat armor pen no longer stacks. No idea why it does, no other pen does. Seems like an oversight.
Solves pretty much all the problems. People that want 54% shred will have to pay for it. Not just buy LW and have a teammate with BC. AD casters never wanted a group utility item, and never wanted a stackable cheese item. They want a single big damage item that'll get them in to the late game.
Neither of these solve the problem effectively.
It merely turns BC into a second LW with more favorable stats - not an item can exist alongside of it. It will either override LW or be useless compared to LW, minus the stacking case on a single champion which forces that single champion to always get both when balanced to the stacked case.
Besides, it's basically group LW as is. Which won't be balanced. Group LW just takes champions like Nasus out of the game because he builds pure tank then you shred off thousand and thousands of his gold investment.