Deathcap tries to fill the same hole as Infinity Edge, for a role where the hole isn't the same shape.
They want higher AP for their scalings, but since all the items have been watered down, the high AP is now locked behind one big stat stick. This is fine for ADCs, whose primary means of damage is a function they all possess intrinsically.
Mages, however, have a variety of abilities, play styles, and they have a variety of items with good effects and passives/actives. They shouldn't have to have the brunt of their late-game power locked into one item.
So I figured, why not delete or rework Deathcap, remove its passive entirely, and instead make it an inherent part of completed AP items? As an example, Hextech GLP-800 would have +80 (+X%) Ability Power, with the +X% part stacking additively across items that have it.
Items that grant more early-game power wouldn't give a %AP bonus in this proposed system. Things like Morello, Luden's, and Gunblade. They trade late-game power for early-game power, like how lethality and crit work on ADCs.
Might need more numerical fine-tuning, but the concept is there.