So Lareth the Beautiful is defeated, the cultists in the Moathouse scattered. Hommlet is safe again. Or is it?
Probably not. The cultists in the Moathouse serve the Temple of Elemental Evil, a classic villain group harking back to AD&D. If one wants to continue after clearing the Moathouse, the Temple is the obvious next stop. I even made a rough plan:
Mid-heroic tier: The characters need to figure out where the cultists came from. The obvious choice is to go to Nulb - a hive of scum and villainy, where one might stumble into recruiters for the Temple. The less obvious choice is to track down the adventurers that defeated the Temple the last time and ask them. It's worth noting that the pair of retired adventurers in Hommlet (Burne and Rufus) weren't part of that dungeon crawl - they arrived in Hommlet later. Plenty of opportunity for adventures in the Big City!
Late heroic tier: Hit the Temple! Make sure to show your players the classic cover (shown above). Grab a pretty map of the first level here! Sadly, the lower levels have caved in (because I like to run with what I have, and Mr. Rivera hasn't got around to drawing maps of the lower levels), with mid-ranking Temple priests running the show on the mostly intact first level. If you want some actual elemental creatures, the Earth Temple should be strong here (with access to Galeb Duhrs, Xorns and Geonids) with the Fire Temple as a fiery minority (and opportunity to use Hell Hounds and Flame Snakes). Mostly humanoids, though. The head priest of the Earth Temple has clues pointing to the For Really Real Now Temple of Elemental Evil (the one that was erected after the old one was razed). Clearly these cultists were just performing archeology.
Capping the heroic tier: Disaster strikes! Hommlet is besieged by an army of giants and whatever else is cool for 10:th level characters to fight. If they break the siege, the PC:s have the full backing of Hommlet (and probably the nearby larger cities of Verbobonc, Dyvers and the kingdom of Celene, if you're using Greyhawk's geography and they work on getting that support). It's time to take the fight to the enemy. Time to hit the Temple like the fist of an angry god.
Paragon tier: This is where all the elementals and archons reside. Time to have some fun with four temples, one of each element. The fire archons are both lowest and highest level - I imagine a plot where they're the strongest in their own holdings and are sending their trash troops exploring the countryside - "random encounters" would be with fire archons. Going with the original TOEE module, each temple has a part of a MacGuffin that can be assembled and used to try to seal away some demon lord. That was Zuggtmoy in the original - as a Nethack player, I'm fond of her nemesis Juiblex (and it makes sense that he would like to take over the cult his rival once manipulated). Kill him to cap off the Paragon tier.
Epic tier: I fleshed this out even less than Paragon tier, but the goal is obvious. The Temple's mortal servants are beaten. Let's take the fight to the planes and the Princes of Elemental Evil - Imix, Ogremoch, Oldydra and Yan-C-Bin. Start with Imix's worshippers among the Efreet and work your way up the hierarchy. Don't stop until you're standing on Tharizdun's dead body raising his severed head above your own.
Imix and Ogremoch gets stats in Monster Manual 3. Olhydra is easily made by taking Solkara from Plane Below and renaming her. Mual-Tar (Dragon 370) or Sirrajadt (Plane Below) make half-decent Yan-C-Bins. Cryonax isn't a classic Greek element and doesn't get to play. You'll probably want to make Tharizdun's stats yourself - or have him be the one guy you can't fight, but have to seal away.
Next up: Oh wait, there's one encounter left in Hommlet!