Monolith realm, an ancient world near covered in city, pretty much everything had been tamed, civilised, or explored. Sort of Shadowrun but pure fantasy. Ancient guilds scheme against each other, hiring adventurers to make runs against opposition.
Sort of magic matures and controlls near everything. Of course a few years later I found out about Ravnica, which is kind of similar concept.

Broken City of Doors: One of the largest cities is laid under seige by powerful magic, the warring wizards tear reality apart in the city. What's left was sealed away for a decade in the hopes the tears would heal. Now the rulers have allowed explorers into this city and found these rifts lead to other worlds and planes... some of which have valuable resources.

Not Cthulhu: Using Call of Cthulhu rules, let the players know that's what it is. Have every adventure actually turn out to be completely conventional in nature with no supernatural elements at all (figure I can probably get about 2 adventures before the players catch on... At this point maybe actually introduce supernatural elements - make them investigate it more like normal investigators would, rather than expecting mythos elements everywhere.