There are a lot of references in D&D to Earth, such as why and by whom the Forgotten Realms were forgotten and Oerth being one of five alternate worlds with varying amounts of magic, Earth being the least magical. So yesterday I got an idea to detail Earth as a D&D Setting, where the PCs would either be visiting from D&D worlds, or being Earth natives dealing with D&D elements crossing over. This has sort of been done before, with d20 modern's Shadow Chasers and Urban Arcana, but I'm thinking of a somewhat different version, including a bit of "Yog-Sothothery". Other inspirations is the 2e Historical Reference series.
The world has had periods of waxing and waning magic, with the earliest having been in prehistory, during the time of Atlantis. The waning of magic contributed to Atlantis's destruction, but magic eventually came back during the Age of Myths, including the age of the Greek heroes. By the time of Classical Greece most magic again disappeared. Magic again peaked during the Dark Ages, including the reign of King Arthur (who yes, exists in this version of Earth), then started to wane again during the Middle Ages, eventually reaching it's lowest point. However by the 20th Century magic again began to be restored, slowly and subtly. By the modern day it has again started to take hold again, and subtly effects society in the background.
Not all myths are true, but they do tend to be partially true. Mythological figures may have had somewhat different motives or be slightly different than they are presented in myth, different kinds of creatures might have been confused for those of a different type, or their may be more than one of a creature that was thought to be unique. The conventional understanding of history is wrong as well, since you know, myths and legends are true.
Most people, even many "bad guys" tend to be neutral, with optional rules for "unaligned" characters and "temporary" alignments, so that characters like paladins don't get completely nerfed. Hitler is evil however.
Just a few of my ideas. I will post more later.