Would it be easier to design Brutalizer upgrades if it didn't have CDR on it?
AD, flat ArPen, and CDR all in one rather cheap item makes it very difficult to make balanced upgrades for it, especially since champions like Riven and Rengar exist.
With the CDR of the Brutalizer, coupling it with another item that grants CDR is iffy. With the recent itemization changes, adding Health to it is also iffy since HP is now fairly easy to come by. And to keep the manaless AD casters from having a field day with it, an upgrade would basically be forced to utilize Mana somehow.
I'm not too good at getting my thoughts out on paper, but here's what I wrote on the matter. Including a redone jungle enchant that I probably really missed the mark on.
The Brutalizer
Attack Damage increased to 30. No longer grants 10% Cooldown Reduction. Flat Armor Penetration passive now named "Sunder", value unchanged.
Essence Reaver
The Brutalizer (1337) + Forbidden Idol (600) + 863 = 2800
+50 Attack Damage (1800) +10% Cooldown Reduction (317) +100% Base Mana Regen (720)
Unique Passive - Sunder: +10 Armor Penetration (120)
Unique Passive: Your basic attacks restore Mana on-hit equal to an amount between 2% and 8% of base Physical Damage dealt based on your Missing Mana. The Mana restored is increased by 50% when attacking champions.
Stat Value: 2957 Gold Efficiency: ~106%
Enchantment - Dominus
The Brutalizer (1337) + Jungle Item (850) + 63 = 2250
+40 Attack Damage +10% Bonus Attack Damage +10% Cooldown Reduction
Unique Passive - Sunder: Grants Armor Penetration equal to 25% of your Base Attack Damage.
Youmuu's Ghostblade
Flat Armor Penetration passive now named "Sunder", value unchanged.
Sterak's Gage
Base Attack Damage increase is now a unique passive.
With that Dominus enchant and Sterak's Gage, a level 18 Skarner would have 43 flat Armor Pen, ~52 during Primal Rage. I'm not sure if that's too much, even if it is at level 18.