The issue with tripling down: Let's talk Brutalizer
Brutalizer is in kind-of a weird spot atm, with patch 5.8 removing its build path option into Black Cleaver. The reason given for that decision makes a lot of sense, as 
is supposed to become an item effective against tanks on a more team-beneficial level. However, this leaves Brutalizer with one complete and one Summoner-dependant build path.
So, what is the problem in designing another Brutalizer upgrade to replace the one that was taken away?
It might have something to do with the item on a conceptual level: Brutalizer is an item that gives three self-synergistic stats for its primary users (AD based casters), effectively giving you a major powerspike, on a rather low cost. This is especially true for champions who only want offensive stats (or need only offensive stats early).
At the moment, I can't picture another item that has that kind of tripling down on on the self-synergy in the same price category (1200 to 1500 gold, first back-items).
Most of these give three effects, but no all three of them lie in the same category (Offensive, Defensive, Utility). Btw, I see CdR as an offensive stat for AP and AD casters and a utility stat for tanks and supports. The closest thing to Brutalizer is Haunting Guise - offensively, it also gives Pen and primary damage stat (AP), but also HP - which is defensive and an unrelated stat to the other two. If it gave CdR instead, it would be in each and every mages build path. Cutlass? Offense (AD/Active), Defense (Lifesteal), Utility (Active) Catalyst? Defense (HP and Regen), Utility (Mana) Hexdrinker? Offense (AD), Defense (MR, Passive) Revolver? Offense (AP), Defense (Spellvamp) Phage? Offense (AD), Defense (HP), Utility (Passive) Armguard? Offense (AP), Defense (Armor) Sheen? Offense (AP, Passive), Utility (Mana)
The only thing that is somewhat similar in tripling down on defense is . Unlike Brutalizer, though, the three aspects on it are not completely synergistic - HP and flat HP regen are both effects that increase your healthbar, but not mutiplicatively so.
The strength of Brutalizer is the reason that upgrades of it usually get crappy buildpaths - old BC had a Ruby Crystal in its recipe and then a 1263 combine cost. Youmuu's, on the other hand, builds out of Avarice Blade, a notoriously inefficient item.
In the end, I think that Riot will be hard-pressed to find another possibility for a Brutalizer upgrade that does not break AD assassins and AD casters completely when rushed.
TL;DR: Brutalizer is the source of its own problems, and most new upgrades will either end up underbought like old Cleaver or hilariously broken on a host of ADs (concidentially, also like old Cleaver during the League of Cleavers).
provides such a massive power spike on its own that it distorts any build path it's related to, which I think is one of the reasons why AD casters have had such a hard time itemizing properly. Ideally, removing it would open up a space for a smaller, cheaper AD+CDR item, which is sorely needed in a lot of recipes and conspicuously absent in an itemization system where nearly every stat can be upgraded to provide cooldown reduction. I also think flat penetration tends to be more trouble than it's worth: larger amounts of flat damage already provide more kill potential against squishies, and don't create weird situations where a stat decreases in effectiveness as the game goes on.
