Lawrence Schick, in his introduction to module S4 in the WotC reprint volume "Dungeons of Dread", tells us this interesting tidbit about the module and how it fit into Gary Gygax's original conception:
"S4 The Lost Caverns of Tsojcanth marked the end of the S series—and rightly so, because despite being based on a gilded-hole dungeon originally designed for a tournament in 1976, its updated version really belonged more to the '80s campaign-setting school of design than to the wild-and-woolly '70s. S1 through S3 were standalone modules that could be easily dropped into any DM's campaign, but Tsojcanth is firmly based in Gary's World of Greyhawk. Indeed, there’s evidence that Gary considered Tsojcanth part of a longer Greyhawk campaign, placing the adventure between T1–T4 The Temple of Elemental Evil and WG4 The Forgotten Temple of Tharizdun. (By this reckoning, The Village of Hommlet, The Temple of Elemental Evil, and Tsojcanth are thus the "lost" WG1 through WG3 modules.) So, Tsojcanth was published in the S series because it got completed out of order, but was too good to delay."(Thanks to Mortellan for pointing that out on his Greyhawkery blog)
Now, this got me thinking of how, exactly, one might transition from the purported WG2 (Temple of Elemental Evil) to WG3 (Lost Caverns of Tsojcanth). If, as Schick claims (and The Acaeum concurs), they were originally supposed to be part of a larger campaign arc, taking adventurers from Hommlet to the Temple of Elemental Evil to the Lost Caverns of Tsojcanth and ultimately to the Forgotten Temple of Tharizdun, just what was the narrative connection?
Now, some of those connections are relatively easy to explain. The Village of Hommlet was specifically written to lead in to the Temple of Elemental Evil. NPCs and plots are common to both, and they are geographically next to one another. That one's easy.
The narrative connection between the Lost Caverns of Tsojcanth and the Forgotten Temple of Tharizdun are a little harder to unravel. Certainly the mechanics of the connection are easy enough; the gnomes in one of the side-treks in Lost Caverns direct the adventurers to the Forgotten Temple to clear out a band of pesky norkers. But in a narrative sense, having the Forgotten Temple as the capstone of the campaign arc seems an odd choice. Perhaps-- just perhaps-- a narrative thread might present itself if we pursue the discussion. Let's put that one on hold for the moment.
That still leaves us with the connection between the Temple of Elemental Evil and the Lost Caverns of Tsojcanth. The module as published isn't much help; the Margrave of Bissel simply summons the PCs and sends them into the Yatil Mountains to find the caverns and loot them. But was there some narrative connection in the original conception as speculated upon by Schick?
I think so, and that connection is Iuz, and two factors point to this as the narrative bridge between the two.
First, according to the Secret History of the Temple (of Elemental Evil) that the whole elemental evil sect was a ploy concocted by the demon princess Zuggtmoy, and that Iuz came on soon after its inception as an almost-equal partner in the scheme. Iuz is also deeply involved in the attempts to free Zuggtmoy and restore the Temple to its former glory, especially through agents such as Hedrack ("the mouth of Iuz" as he is known) and Barkinar.
Second, the Lost Caverns were of import precisely because they were used as a base by the witch-queen Iggwilv. It is her magical treasures that the Margrave of Bissel wants to keep out of the hands of his enemies in Ket, in particular the Lanthorn. And, of course, Iggwilv is the mother of Iuz.
Now, this is all speculative, but building off Schick's statement that some sort of meta-narrative was originally planned for the series, Iuz becomes the narrative connection between ToEE and LCoT. That connection was never, obviously, written into the modules as published, but we can speculate based on what is there.
We know from ToEE that Iuz is trying to help Zuggtmoy escape imprisonment under the Temple and generally help the Temple itself to rise. From LCoT we know that Iggwilv, the mother of Iuz, was known for her deep knowledge of demons and demonkind, as well as her own stockpiles of magical might.
Perhaps Iuz believes that something his mother had could help free Zuggtmoy. This could be a hitherto-unknown spell in her spellbook, a function of Daoud's Wondrous Lanthorn, the aid of some demon or other extra-planar creature that Iggwilv had in her thrall, etc. The PCs, having (presumably) stopped the initial attempt to free Zuggtmoy in the Temple of Elemental Evil, would then set off to stop the agents of Iuz from getting access to Iggwilv's treasure. In essence, Iuz replaces Ket, and Veluna/Furyondy (perhaps even the kidnapped Prince Thrommel himself) replaces the Margrave of Bissel.
One could also tie it in with a return of Iggwilv, who is looking to assist in Zuggtmoy's return to power and thus use her as a counterweight to the power of her own son, who is naturally more difficult to control as his own temporal power grows. That's similar to what we see in the Gord the Rogue books, by the way, but in this case the Theorparts would be replaced by the various magical devices and so forth left behind in the Lost Caverns of Tsojcanth.
As far as the Forgotten Temple of Tharizdun goes, the parallels between Tharizdun and Zuggtmoy, as published, are striking. Both are powerful evil entities that are trapped, with followers attempting to free them without success (Tharizdun's followers having given up, while Zuggtmoy's are still at their labors). If we want to see a narrative connection between ToEE and FToT, perhaps it lies in those very parallels. What if, in a bid to help them free the Demon Queen of Fungi, the followers of Zuggtmoy are looking to get the secret of the Daily Ritual of Awakening, thinking that where it failed with Tharizdun, it might work with Zuggtmoy!?
Again, just speculation, but those sorts of changes would not be at all difficult to make, and would allow an enterprising GM to turn the modules into the sort of campaign arc that, if Schick's statement is to be believed, was originally intended before the realities of publishing interfered with Gygax's original plan.