Here's the tough thing. I really like Kyuss as a god and I really love the Age of Worms adventure path--enough that I would like to set a game in its aftermath.
So Kyuss "dies". What does that mean in Greyhawk parlence? Does he go back to his plane to reform himself? Does he go off to rot in the Astral Plane and have githyanki build summer homes in his ribcage? Is he just gone?
My hope would be that this is not the last we ever see of him since he really is one of those badguys who's near and dear to my heart.
Of course the only other way for things to go down leads to the age of worms and a postapocalyptic version of Greyhawk. Not real interested in that road either. I guess what I'm hoping for is that Kyuss has to pull back from his plans and brood and scheme--evil god screensaver mode if you will, just ambiently doing things in the background for a good long while.
Though regardless what I really want to know is the official/semi-official/guys opinions on his ultimate fate after the big showdown with him.
Well, Kyuss IS a god. Deities & Demigods has several ways to treat the mortality of the immortals. The default assumption is that if a character can successfully "kill" a god by whatever means (treating it as nothing more than another creature with some hyped up special abilities), the god is *dead,* forever (though I suppose a god of higher rank could bring a dead god back).
I tend to like something about what you're talking about - evil-god-screensaver-mode. IMC, gods are truely immortal and unkillable by any means - unless they decide to attempt to interact directly with the Prime Material, in which case the "avatars" they send (the god's full statblock, or an avatar or aspect) can be temporarily slain, death meaning banishment for a somewhat variable span of time.
Even killing a god's physical form on its home plane would not be enough - though it would severely extend the time of banishment.
TK
Well, you could make him like Sauron in Middle Earth. Kill him in his current form and it seems like he's dead, but his spirit is powerful enough that it doesn't just dissipate--eventually it reforms somewhere. Maybe the taint in the bottom of the caverns created by the monolith's collapse continues to exercise some sort of malign will. Like the Malgoth in Greg Vaughan's Istivin arc, it is somehow bonded to this place, and eventually it seeks out servants it can dominate, to work its will. This makes for a cool follow-on campaign set ca. CY 615, when the PC rulers of Alhaster are getting on in years and have to hire some young adventurers to find the source of the curse that inhabits Alhaster.
Well there are many ways to deal with the death of a god.
Killing Kyuss released a bevy of divine energies, divine energies which dealt with the creation and control of undead. Perhaps a follow up campaign dealing with someone trying to find and harness the power of Kyuss.
Perhaps the person doing this is a misguided individual who believes they could handle such powerful energies responsibly.
Perhaps it is a powerhungry lichlord or vampire trying to find something akin to true immortality.
Maybe its a race between various factions and the heroes have to take sides.
I think bringing Kyuss back again sort of cheapens the victory at the end of the campaign. The best bet is to create a new threat based on his divine energy.
You could have him infest one of your characters, particularly one of an arcane or clerical bent. He hides a fragment in the player's soul, slowly corrupting away at him until he can find a way to re-awaken as a deity (a la Blizzard's Diablo series, or Final Fantasy: Advent Children).
He could also become a vestige, using the rules from WOTC's Tome of Magic; a banished being unable to truly act upon the world unlessed called upon by powerful (and foolish) mortal binders.
I'm not sure how Greyhawk usually handles this sort of thing, but in the Forgotten Realms, some other god would just take over Kyuss' portfolio.
If that works for Greyhawk, you could even have it be someone new rising to godhood and taking his portfolio. My personal choice would be Mak'ar, the Harbinger of Worms from Spire of Long Shadows (so what if the adventure said he didn't have a phylactery and when he died, he was dead-dead; the PCs can't know that for certain, and maybe he planned this all along as a way of getting Kyuss to create the portfolio so that he, Mak'ar, could eventually usurp it).
Then Kyuss himself can become a Vestige, but he may try (or someone else may try to help him) to reclaim his godhood and divinity at some future point.