Mechanic/item idea; Scaling and % magic penetration

Similarly to how Armour Penetration and Lethality work, there could perhaps be a similarly differentiated penetration mechanic for AP items as well, such as Magic Penetration and Salience.

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Magic penetration would mirror Armour penetration with percentage reductions and build off of one base item along the lines of;

+ 565g = 1,000g

+20 Ability power Unique – Attunement: +10% total magic penetration.

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Salience would mirror lethality with flat, scaling penetration and have a core item built along the lines of;

+ 350g = 1,200g.

+40 Ability power UNIQUE: +10 Salience (6 − 10 (based on level) magical penetration) UNIQUE - Headhunter: After killing any enemy, your next damaging spell will deal 30 bonus magical damage to all enemies it hits (30 second cooldown).