The entirety of GAZ3: The Principalities of Glantri for basic D&D, including:
- Schools of magic as actual schools/secret societies, with initiations and ranks
- Rival noble houses of magic-users, secretly infiltrated by families of werewolves, vampires, traitorous elves, and at least one radioactive lich(!)
- Settings built to generate crazy numbers of adventures, like a school of magic filled with intrigue and peril (detailed when JK Rowling was still a starving college student) or a capital city built over the power-source of an ancient craft, which has become a path to godhood for wizards powerful enough to master it
- Crazy magic items like a tiny alchemical laboratory in a battle, from which you remove your homunculous with a pair of tweezers to enlarge the magic potions he's made for you
- Princedoms of magic-user supremacists who kill clerics on sight - one of which is ruled by magical colonists from Clark Ashton Smith's Averoigne setting (aka the medieval French wizard family from the classic module Castle Amber)
- Glorious Stephen Fabian artwork - it is crazy to me that the guy who created the look of Ravenloft no longer draws for D&D
None of that is system stuff, but it is all part of a design philosophy where anything was possible and nothing was too outlandish to fit within the rules.