AD Assassins get Lethality items focusing their stealth and mobility with an EXTRA punch to their attacks (Hi, Duskblade) and even cover from hostile spells (Oh hey EoN). AP Assassins get access to Mage items, so sure they get Banshee's veil, but. What good is MR against an AD assassin. Or a spellshield against a Luden's.
AP Assassin's should have item options built around penetration, and Mages should be the ones stacking high AP values. They're MAGES. Katarina doesn't cast magic, Akali doesn't cast magic, LeBlanc. Ok well she needs some more help anyway. Fizz is just splashy. None of these characters use magic, but they have to build the same AP items that Mages do. It doesn't feel right. And if you're thinking, more magic penetration will just make Mages busted, look. Mages are already in a great spot right now and they can all one shot you. That Luden's shift helped a lot. But it hurt the Assassins. Akali and Kat namely who don't benefit from Mana. Even Lich Bane on Akali feels like wasted gold because, yes, even though it flows in her kit, it builds from a Mana item. Mana, widely recognized in every fantasy game ever as a character's resource to cast spells. Which AP Assassin's don't do. They dash, and blink, and throw knives, and feed you to sharks. Evelynn I see you, too.
So Lower base damage values on Mages and increase their AP Ratios to reinforce that class. Up base values for Assassins so they can stack penetration to annihilate that Marksman like they're supposed to. The only middle ground between the classes should be Void Staff, Rylai's Crystal Scepter, and Zhonya's Hourglass.
New Core Item: Ceremonial Blade (Amplifying Tome + Amplifying Tome) - +X AP, +Unique 10 Magic Penetration , + X Out of combat Movement Speed.
Lich Bane: (Ceremonial Blade + Blasting Wand) - +X AP, +15 Magic Penetration, + X OoC movement speed - After using an ability, your next auto attack will deal X magic damage on-hit.
*New* Blood Moon's Claim: (Ceremonial Blade + Fiendish Codex) +X AP, + CDR, + 15 Penetration, + OoC movement speed - After using a dash, blink, teleport, etc. your next ability against an enemy champion will deal 20% bonus true damage. (Most mages cannot access this item. Protobelt should not be able to proc this)
*New* Shadow's Grace: (Ceremonial Blade + Ruby Crystal + Null-Magic Mantle) +X AP, + X HP, +X MR + OoC movement speed - After a takedown, gain a spellshield for 2 seconds and 30% decaying movement speed for the duration. (Their ticket to getting out after an all in, or an incentive to keep going/Made Unique from Banshee's Veil).