OP, did you ever tried to implement DFG into your build with the icnreased dmg? Cause every dmg will be worth 20% more (scaling and base dmg) for 4 seconds on the target you want to burst. Best AP single target burst item in the game by far.
The loss in AP you are feeling is purely Rabadons. But every late game item got nerfed by a good amount. Buffing the normal AP items would give AP champs a too strong mid game.
Sure, i think there are items that need slight buffs (Morellonomicon should get +10 AP and some other items +5 AP). Athene's is not one of them. This item gives nearly infinite mana reg and a good amount of MR. It shoulnd't give also a lot of AP. It is more of a defensive spam skill item for Orianna and not a high AP item for Annie or Brand.
Now about other items:
And compared to the ArPen, MPen didn't get hit as hard (10% nerf on runes and 20-25% nerfed on items; Abyssal still 20 MRed).
ArPen got nerfed by 20-25% on runes and 20-33% on items.
When you look at end game items, you don't find one that didn't got nerfed (except for the OP bullwark, what is too good, but was not there before).
For AP items, only the OP Athense got nerfed (when every mid laner rushes this item at the end of S2, then somthing is wrong with it) and Rabadons.
DFG got buffed.
Rylais got buffed (HP is a better stat now cause all other defensive stats got nerfed).
ROA buffed (same as rylais).
Dorans got also nerfe a bit (dont stack it anymore).
Haunting Guise and Torment are OP currently.
In the end every champ got a dmg or survivability nerf.
ADC got an overall dmg nerf right when they are finishing PD or IE (or TriForce).
AP got a dmg nerf when finishing Rabadons.
Bruisers and Tanks lost survivability.
AD casters are the main winners and also some AP assassins. They are finally good again. But comming out of an UP status doesn't mean they are OP (Korea is already back to the normal meta with some support exceptions).