I think betting on any of the bad guys getting it might be looking at long odds.
Putting aside the fact that Arazni is categorically less-awful than just about any of them, the event isn't really being presented as the cause for celebration most of these dirtbags getting it in the neck would be.
Of the bad guys left, whose demise would make for a good story?
Azzmatazz and Urgathoa are both okay. That just leaves Nordberg, Zonny-Boy, Rover, and Lamashtu.
Norgobbler isn't one I'd miss (I find him more useful than Cayden, but it's marginal), but the sheer number of bad guy cults he can sponsor while being a wholly-owned bit of Paizo IP... and his death? Given his whole secretive schtick, he'd be like friggin' Alpharius in 40k...
"Ah, but is he really dead?!"
"Pretty sure, seeing as I ripped his head off."
"But was that really him?"
"You know what, I don't even care anymore."
Zonny-Boy feels like most of the gas is left in his villainous tank. His whole story is the mystery of what happened to him to turn Dou-Bral into Zon-Kuthon. Unpicking that feels like a more rewarding story thread than in just tossing his deific corpse into the stratosphere and bathing in the fallout. He and Shelyn are unresolved deific business, and with all due respect to the merge theories out there, the relationship is most interesting with them alive, opposed, but Shelyn holding out hope for her brother's redemption.
Lamashtu's whole deal is just so neat and omnipresent that I'm having trouble imagining a scenario where her death is more interesting than her survival. She's also a divine murderer of record, making her a natural suspect whoever else gets it in the neck, and I just don't see wasting a perfectly good red herring as wise.
Rovagug is loaded with importance as the guy so bad he could get Asmodeus to play a relatively straight hand with the other deities. Plus being presented as so insanely powerful that the gods banded together to imprison him... I...
I think it may have been categorically stated that we'll never know what is possessing Zon Kuthon.
It's similar to devourers. Any answers you provide make it way less creepy. And Paizo likes leaving open areas for GMs to fill in.
I agree with you that the death is not being presented as a good thing though...