Black Cleaver is too easy to exploit
The nerfs coming with the tank itemization changes will not solve the real issue with Cleaver being too general. It's actual issue is how fast certain champs can stack it. Graves, Lucian, and Ashe build it for one reason: multi-hits. Graves can fully stack it in one crit, and Ashe and Lucian can nearly do the same. They would normally build Last whisper, but with instant stacking they might as well take Cleaver.
How do we fix that? Put a cap on how many times per second it can stack. I think a good number would be 2 times per half second. That means that the minimum time to fully stack it is 1.5 seconds, but realistically even the abusers would take around 2 seconds. The intended champs would not be affected so much.