Sounds like too much, and not enough.
1. Stacking BCs will still give me ample AD, CDR, flat pen, AND HP. It doesn't seem this is changed, unless all the brutalizer and derivative flat pen becomes unique, rather than just brutalizer not stacking with the others.
2. You're actually nerfing early game power of champions quite a bit.
3. You make it harder to actually make arpen worthwhile, it's becoming worse than season 1, for most champs, and the ability to stack penetration and remain squishy is something that should be open to assassins (I mean, where would Leblanc be if that weren't an option?).

So, it seems to me the issue of BC itself isn't really addressed that much, but the issue of stacking penetration as a squishy assassin had been taken too far.

If most of the stats on the item were unique then it wouldn't be a problem whatsoever and this thread wouldn't need to exist.

1. It's ok if Brutalizer flat arpen stacks with Black Cleaver or Ghostblade. Additional arpen items makes champ less itself and it should be kept like that.
2. I don't like % penetration on other item than Last Whisper at all, not to mention that many champions already have skills that greatly reduce % armor. Either it should be flat reduction (f.ex. 10 armor reduction that would stack 4 times, making it -50 armor overall with BC nerfed flat armor pen from 15 to 10) or % reduction, but one choice option between BC and LW (that means you buy BC = you cannot buy LW anymore, you buy LW = No BC)

I see Black Cleaver more or less like that:

Since price of Brutalizer is 1337 which means "leet" I would see two versions of it.

1. Overall price of BC: 2493 (combine cost 681) which means "rage" and for that you'd get:

+250 hp
+35 ad
+10% cdr
+15 arpen
Unique passive: Dealing physical damage to an enemy champion reduces their armor by 5% for 4 seconds, effect stacks up to 4 times. This passive loses it's effect if you have Last Whisper in your inventory.

2. Overall price of BC: 2107 (combine cost 295) which means "Riot" and for that you'd get:

+200 hp
+25 ad
+5% cdr
+15 armor pen
Unique passive: Dealing physical damage to an enemy champion reduces their armor by 10 for 4 seconds, effect stacks up to 4 times.

I love the item, and think it's a nice addition to all game modes it's in.

EXCEPT - The stacking needs to all go away.
It's not interesting or fun to see people stacking BCs for massive Arm. Pen.
It's a boring, simple, and mechanic abusive way to build and play a game that has so much more to it!

BC basically destroys all build diversity. Even with proposed nerfs stacking 5 BC's is still better for some champs.

With those nerfs Cleaver wouldn't be a problem on Dominion.

Though I think nerfing the flat armor pen AND the armor shred is a bit much. Drop the armor shred. Keep Whisper as the big percentile item of choice. Probably a decent idea to drop the Ruby Crystal and replace it with a Longsword 'n up the AD to 60.

I still don't understand why it has HP on it, in the context of 'big ticket items' for damage dealers.

Xypherus pretty much implied that it will never happen, since in doing so, it will make a direct competition between LW and BC.

I agree with Sauron about the removal or unique HP.

i like this actually if the health became an unique passive then it would be fine as it is, stacking the shred would be strong at shredding but it wouldn't me the ultimate tanky anti-tank item.

I thought it was usually the rule in that returning passives from upgraded recipes were unique or non-stackable with their previous separate items. I think it would be good to make BC function similar to Brutalizer or Ghostblade in retaining similar stats as uniques for starters. Bruisers were always strong before the changes, and BFT did receive a nerf so things should be more balanced without the stacking.