Honestly, executioner's calling should build into something else...

Being able to build into makes it too easy to shut down an armor heavy champ that relies on sustain. The ones building are, 90% (leaving 10% for those "certain" picks) of the time, damage dealers who have the damage to kill a tank.

Sustain is the niche for certain champs and they build armor so that they can live long enough to sustain somewhat against those heavy physical damage champs. But when you give a full damage champion the option to get and armor pen in the same item, it completely closes out an entire class of champions in a single item slot.

As far as ideas go, perhaps make build into an item that also gives lifesteal, or an item to make it appealing to different types of damage dealers.