I was a huge Robert E. Howard fan before I discovered H. P. Lovecraft and CoC, I was already into roleplaying, and I did an Atlantis scenario years ago. I located mine in the Canary Islands and used a ziggurat "airport" instead of the typical pyramid. Atlantis was essentially the domain of Howard and he had written that they were incredibly advanced with multiple variants of oricalcum, crystal based technology/ magic and even flying ships, hence the airport aspect of the ziggurat. In my adventure it was more of a D & D dungeon crawl through a half dozen levels filled with Deep Ones and hybrid Atlanteans and I've always wanted to revisit it but have never gotten around to it. You could incorporate something like that where they can actually explore a massive, but mostly intact, ruin before finding a handful of survivors and then meeting the climatic monster, maybe the Atlanteans have been making human sacrifices to appease the creature. This could lead to many interesting possibilities like learning Senzar, ancient Atlantean technology or spells, finding an antediluvian library filled with sanity shattering scrolls, ect, ect, ect. LOL