You mean all Greyhawk has is Tiamat, the Temple of Elemental Evil, and whatever two or three other things WotC might use for their follow-up adventure books? After that, Greyhawk is devoid of anything else useful to put in a campaign book and there's nothing new they can create or add?
It's a lot worse than that, as Greyhawk doesn't really have Tiamat or Elemental Evil!
One of the important things to understand about Greyhawk is that, although it's the origin of a lot of D&D material, the link of that material to the setting is often not as strong as you might expect. The Queen of Chaotic Dragons" appeared in Supplement I: Greyhawk (1975), and so might have been part of the early D&D games when Greyhawk was the only setting - most of what appears in "Greyhawk" is considered generic D&D. She is first actually named "Tiamat" in the first Monster Manual (1977). She only gets one passing mention in the Babylonian mythos section of Deities and Demigods - apparently Marduk's battles with Tiamat are legendary - and is listed as a lesser god in the Nonhumans section with no further details forthcoming.
She got raised to the rank of deity by Len Lakofka in Dragon #38 (published about the same time as Deities & Demigods), and at that point she basically disappears from things related to Greyhawk (except for a couple of modules by Frank Menzter which aren't normally considered Greyhawk adventures).
Tiamat's doppleganger, Takhisis, appears in memorable fashion in the Dragonlance Saga, and that's the biggest influence Tiamat has had on the game.
Meanwhile, Tiamat makes her first appearance in a Forgotten Realms product in H4: The Throne of Bloodstone (1988), where the players are told that the Wand of Orcus can only be destroyed by bathing it in her heart's blood. (And yes, the PCs do encounter her). She pops up in other Realms products from time to time, in particular the Draconomicon (1990), which places her very firmly in the Realms.
All of which is a long way of saying that Tiamat is part of "generic" D&D - the core underpinnings of the game which we then put setting material on top of. There's no special connection to Greyhawk at all, and she's certainly never really been part of any important Greyhawk adventure or product.
On the other hand, Elemental Evil...
The Temple of Elemental Evil is a core part of Greyhawk mythology. In fact, its one of the few bits of Greyhawk mythology we have, and one of the few important sites that gets a mention in the "Brief History of Eastern Oerik", which appears in both the 1980 Greyhawk folio and the 1983 boxed set. Of course, it's mention in the Village of Hommlet (1979), but it would be five years - and after Gary left the company - that the actual site itself was detailed (1985), with the manuscript finished by Frank Mentzer.
However, it's not the only Elemental Evil. In 1981, the Fiend Folio came out and amongst its monsters were the five Princes of Elemental Evil, designed by Lewis Pulsipher, one of the significant figures of gaming. Interestingly, it seems that the manuscript of the Folio may have been completed in 1979 - for Games Workshop - before it eventually got published as the first product of TSR (UK) (see
the Acaeum).
The Princes of Elemental Evil have hardly been used. Monte Cook decided to put one in his Return to the Temple of Elemental Evil, which made sense - sort of - but the original adventure mentions them Not At All.
And here we run into one of the conceptual problems of later developers in Greyhawk. The original story for the Temple probably involved Lolth. Then things got changed around, and the published product has Iuz and Zuggtmoy setting up the temple. Well, mostly Zuggtmoy, the Demon Lady of Fungi. Why Elemental Evil? Because she thinks its more attractive than Fungi to worshippers. She probably had a point. Blasting people with (evil) fire is a lot better than feeding them mushrooms. But there's no real *power* of elemental evil in her worldview. Instead, she's just harnessing the power of the elemental planes to evil ends. This is in sharp distinction to Lewis Pulsipher's Princes of Elemental Evil.
At this point we have the Greyhawk conception of elemental evil against the generic D&D conception of the Princes of Elemental Evil. They're two different things.
And then we get to the complications. Gary Gygax included in his D&D adventures two rather unusual deities. Tharizdun and the Elder Elemental God. They were not intended to be the same thing. Here's Gary on them from Oerth Journal #12:
"The Elder Elemental God I saw as a dark creative deity, one that spun form out of chaos in his portion of one universe, then lost control of his creation--as is the story with so many deities of this sort in the mythology of various peoples of earth, from Babylonian and Egyptian on. Tharizdun is a larger and more pervasive force that is multiversal but not omnipresent. That is what he sought, of course, along with omnipotence. Tharizdun failed on both accounts."
"The EEG was indeed meant by me to have a place in the very nethermost recesses of the ToEE. An anomaly there allowed him to manifest a portion of himself, and by doing the wrong (right from the DM’s point of view) thing the adventurers could release him also! Of course that would counter somewhat the freeing of Zuggtmoy, had she been loosed, so on balance it could serve to redress that error. But, alas, I was too busy with other things at the time when the project was being completed. As it was already quite hefty, I decided to omit any mention of this to Frank Mentzer, and so the ToEE was released with only the Eye of Fire as a clue to what I should have included in the adventure."
The Eye of Fire is the symbol used by the elemental cultists, which is also the symbol of the EEG in the Giant/Drow adventures. (It's misdrawn in the Hommlet adventure...)
Of course, Monte Cook decided that the EEG and Tharizdun were one and the same, and so wrote in RttToEE a story where Iuz and Zuggtmoy were the dupes of Tharizdun, who was using them all to get freed... Thus, the Elder Elemental Eye turns up. And lovers of Gygaxian Greyhawk get very annoyed!
However, note what Gygax uses to describe Tharizdun: "multiversal". It seems fair to have Tharizdun turn up in the Realms when he's described as such... unfortunately, we have to deal with the conflation of him and the EEG.
(In fact, this use of Tharizdun is some way from the original Greyhawk conception of him...)
Cheers!