Brainstorming: Rabadon's, AP jungle enchantment, and a new component item.
Bound Grimoire
Recipe: Amp Tome x2 (870) + 330 = 1200
+40 Ability Power
Unique Passive - Arcanum: Increases your Ability Power by 10%. For every 1 Ability Power gained this way, you also gain +1% Cooldown Reduction, up to a max of +10%.
Enchantment - Magus
Recipe: Bound Grimoire (1200) + 425 = 1625
+60 Ability Power +10% Cooldown Reduction
Unique Passive - Arcanum: Increases your Ability Power by 25%. For every 5 Ability Power gained this way, you also gain +1% additional Cooldown Reduction, up to a max of +10%.
Rabadon's Deathcap
Recipe: NLR (1250) + Bound Grimoire (1200) + Blasting Wand (850) + 500 = 3800
+120 Ability Power +10% Cooldown Reduction
Unique Passive - Arcanum: Increases your Ability Power by 35%. For every 14 Ability Power gained this way, you also gain +1% additional Cooldown Reduction, up to a max of +10%.
The 'new' Magus and Rabadon's each grant 13% CDR by themselves, between their innate CDR and the base value of their Arcanums. Fun with unlucky numbers.
Basically, an AP branch of items with the Essence Reaver functionality, translating a combat stat into CDR. Arcanum only grants a maximum of 10%, though, because AP itemization still has other CDR sources, most notably Athene's/Morello and their 20% CDR.
Mostly because Rabadon's being tied for most expensive item in the game and granting only ONE stat is kinda... silly? By comparison, Trinity grants seven stats and two unique passive effects. The slightly cheaper Mercurial grants three stats AND a useful active that also grants a temporary fourth stat. BT grants two stats and a passive. IE grants three stats (critical strike damage is a stat, since runes exist for it).
Like the title says, though, it's just brainstorming.
Edit: Adjusted the buildpath on Deathcap. The items in it add up to 140 AP, but the Deathcap itself grants 162 with its own passive anyway.