The idea piggybacks on Aura of Despair, either as extensions of it (like enervating spirits) or as alternatives. It's kinda cheesy, but what about auras of specific evil forces/demons/spirits, ala the Seven Deadly Sins (or... something).
An aura of Wrath, perhaps? Maybe autoattack good/neutral priests, bonus to hit/dam. Aura of gluttony? The AP's teeth grow into huge, demonic flesh-shredding fangs, and they take random bites out of foes, but if they kill something, they automatically hunker down and start feasting on their kill's body/possessions. Lust? Maybe helps sleep land? Envervating spirits takes care of Sloth. Et cetera. Greed, Envy, and Pride are a bit harder to think of immediately.
I know the sins are done to death, but maybe give AP's more tactical options that tap into demonic power? *shrug*

Aura of despair is a breeze of evil spirits shrouding the A-P, not some partial shift. Your suggestion has some idea of more versatile channeling of the spirits provided by Aura of Despair, but no Aura of Gluttony to give AP fangs etc(fangs are not an aura, but maybe spirits could be channeled to form them?).

Sure, it's not a partial shift. I'll call it a "change" or a "transformation" caused by a controlled possession of the AP's body by the naughty spirits.

Semantics, semantics!

In this case, your suggestion would work better as an additional casting option of enervating spirits instead of additional "Aura of Gluttony".

eg. c 'enervating spirits' gluttony
the blows could give some kind of famine like affect instead of sloth, as the spirits beckon with gluttony in this case.

Or it could be a random affect.

I only know about APs from what I read about them. Apparently they need to be loners to grow in strength. So why not have Pride force people in a group fighting the AP to find it even harder to hit him or maybe even be driven out of the battle?

Would that be a good idea?

Greed could give the AP exclusivity to the corpse of anyone he slays? ie Only he can loot till he releases the corpse to the rest of the players?

But I'd probably make ap's choose one to the exclusion of others.

That'd be a neat little feature, and could be useful for roleplay, too. You can choose your favorite brand of evil. Har.