AP Diver / Vanguard Item
A thought for an AP Diver / Vanguard item:
Force of Nature (2900g) for the sake of being unoriginal

Amplifying Tome (435g) + Seeker's Armguard (1100g) + Negatron Cloak (720g) + (645g)
+50 Ability Power +45 Armor +45 Magic Resistance UNIQUE ACTIVE: Deals 70 (+15% AP) (+5% bonus health) magic damage to nearby enemies and slows them by 20% for 1.5 seconds. If 3 or more enemy champions are nearby, you gain a shield for up to 2.5 seconds equal to 210 (+45% AP) (+15% bonus health).
Grants 2797.5g worth of stats, making it 96.5% gold efficient without its active.
Late game with, for example,
you'd get:
Bonus health : 300 + 350 + 500 = 1150
AP : 60 + 100 + 50 = 210
UNIQUE ACTIVE: Deals [ 70 (+31.5) (+57.5) = 159 ] magic damage to nearby enemies and slows them by 20% for 1.5 seconds. If 3 or more enemy champions are nearby, you gain a shield for up to 2.5 seconds equal to [ 210 (+94.5) (+172.5) = 477 ].