How about: To Slay A God?
The AP would consist of the PCs gradually building up to finally destroying Rovagug once and for all. They'd do it piecemeal- destroying enclaves of cultists, artifacts and relics sacred to Rovagug; eventually destroying his remaining spawn. Then, when he's at his weakest, they'd go into the Pit of Gormuz itself and finally eliminate Rovagug in an epic confrontation.
(Among other things that could be done with this AP, we might finally get more info on Casmaron.)
Seeing as how a coalition of all the gods couldn't pull that off, I think that's far beyond the scope of what PCs can be expected to accomplish.
However, setting the same sights lower, what about, say, Achaekek? He's a legitimate deity, yes, but a minor one, and removing him could set the stage for the matriculation of a far more interesting and dynamic figure into the role of patron of assassins and thieves: Nocticula.
Because, frankly, Achaekek is boring and his assassins turning into giant insects is more silly than intimidating.