The first question is, who should this hypothetical 120 AP +10% CdR - item be strong on? Assassins, mana-users, waveclear mages, immobile burst mages, tanky APs (probably not), glass cannons...
We currently have 3 big-ticket AP items in the game:
,
and
.
Of those three, Deathcap is the quintessential stat stick - gives 120 AP and then MORE AP.
Zhonya's possesses some defensive stats (50 Armor) and a unique defensive/utility active that allows you to survive while waiting for cooldowns or an over-time effect like Slicing Maelstrom or Soul Shackles, but that has the downside keeping you in this state for its duration, for the better or worse.
Seraph's gives you a lot of mana to work with, as well as an anti-burst defensive based on your own mana.
So, we have pure offensive stats once and two times some kind of defensive utility. What if the new item had some offensive utility? Meaning, a slow (because harder CC on an item is a no-go). A shortish, possibly targeted slowing effect that enables you to keep enemies in range of your spells for longer or allows for easier pursuit, while not retaining DFG's problem of facilitating faster 100->0 no counterplay burst.
However, we can't just use Twin Shadow's for that - that item's active is more general utility, as it not only slows targets, but, more importantly, also chases them out and reveals them on their own accord. Our item's purpose would need to be more limited and to the point, as its power budget needs to be split between the stats and the active.
Also, Twin Shadows features movement speed, too - which is one of, if not the most, valuable stat in the game (why else would we always dedicate a whole item slot for an MS-focused item?).