No Hybrid Penetration item?
I'm not sure why it took me so long to notice this, but there is no item that provides mixed damage penetration. Whether this is an intentional choice to make mixed-damage champions have to pick one side over the other, I still think it would be worth looking into. Perhaps one that provides a bit less on raw offensive stats to compensate attacking from two fronts.
One I conceptuallized is something that applies a sort of deathfire touch DoT effect on-hit
Blindbrandt
Recipe:
+
+1200g (total 3150g)
+25 AD +45 AP Unique: +15 Lethality Unique: +15 Magic Penetration Unique: Hitting an enemy with a basic attack burns them for 6-40 (Based on level) (+24% Bonus AD +8% AP) magic damage initially and 2 (+12% Bonus AD +4% AP) magic damage every second for 3 seconds after
(1400g) +
... but the real problem for this item is 