If you were going to combine Nashor's with something, Lich Bane would make more sense, I think. They're both AP auto-attack based items with relatively few champs using them as a "core" item. (One of the champions Nashor's is core on doesn't even buy it for the passive...)
I'm not saying it's a good idea by any means, just that it would (arguably) make more sense than Nashor's + Wit's.
Recipe: Sheen (1050) + Stinger (1100) + Aether Wisp (850) + 600 = 3600
+70 Ability Power
+40% Attack Speed
+5% Movement Speed
+250 Mana
Unique Passive: +20% Cooldown Reduction
Unique Passive: Basic attacks deal 15 + (15% AP) bonus Magic Damage on-hit.
Unique Passive - Spellblade: After using an ability, this item's bonus Magic Damage on-hit for your next basic attack is increased to (75% Base AD) + (50% AP) and can be applied to structures.
An item giving auto-based APs a bit of everything they need, like a magic Trinity Force. Again, though, it's not particularly a 'good' idea. Especially since Diana exists.