"Discover the truth about the great conflicts of the D&D multiverse in this supplement for the world’s greatest roleplaying game.
This tome is built on the writings of the renowned wizard from the world of Greyhawk, gathered over a lifetime of research and scholarship. In his travels to other realms and other planes of existence, he has made many friends, and has risked his life an equal number of times, to amass the knowledge contained herein. In addition to Mordenkainen’s musings on the endless wars of the multiverse, the book contains game statistics for dozens of monsters: new demons and devils, several varieties of elves and duergar, and a vast array of other creatures from throughout the planes of existence."
So let's get this out of the way, this is a multiverse book not a Greyhawk book. I highly doubt it has anything remotely to do with "foes" of Mordenkainen. We do know from 2E Mordy likes to study planar subjects though so I fully expect alot of old 2E era Planescape, Blood War type stuff to appear in this. The elf and duergar race bit is probably the stuff they've already been playtesting via their Unearthed Arcana articles. They claim its not a Monster Manual, but it feels like one, or at least half-monster/ half-planar guide which wouldn't be bad. I would also hope there is some cool magic items. Can't go wrong there. So yes I'll buy this book and not just because the cover has Mordenkainen shilling for it (I never bought 4E Mordenkainen's Magnificent Emporium).
Not so idle thoughts beyond the book: what does this mean for the DMsGuild and content creators yearning for Greyhawk? Keep an eye on that site for next several months I guess. If I see Mike Mearls at Garycon in March though I'd be keen to ask if having Mordenkainen's name slapped on the cover makes him viable to use by DMsGuide rules? (not to mention this is his second 5E appearance, ahem) I've mused Tales From the Yawning Portal could be a loophole for Greyhawk creators due to the books content, so if not then, surely now with this book's cover? How much Greyhawk can they tease us with and still keep it buried? Let's hope Tome of Foes is a groundbreaking publication in more ways than one...