How about an AP QSS item?

I agree, something to allow my Lux or other mid-lane mages to counter a Warwick or Malzahar, whom Zhonya's doesn't work against.

No, is powerful enough without being able to completely negate CC without compromising her build path with

I guess they feel AP gets Zhonya's and AD gets Mercurial Scimitar, but a QSS + AP item could also be nice...

If you want QSS as an AP, get QSS.

The main advantage brings is combining a and in the same item slot, otherwise there isn't much difference between having a and in your inventory.

An average game only lasts 33 minutes, seeing ~11,000 gold per player. Average gold per mid-lane champions is currently 11,877 according to LoLKing ATM. 11,000 works out to three major ~3,000 gold items, ~1,000 in boots, and ~1,000 in the bank when someone's Nexus explodes after that final team-fight, or a similar value taken up by the post-game bank and lanning items/consumables.

In short - you're not actually hurting yourself if you divide item slots into a QSS/major AP item dynamic. You can even upgrade that Needless into a if the game drags on longer than normal, and if you actually do run out of space - QSS only sets you back 375 gold if you need to sell it for 70% of it's cost.