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Something original... like vampires and werewolves
How about with 5e, Monster Manuals are organized by challenge level. MM1 would cover an assortment of level 1-5 beasties... so the cover should be kobolds, skeletons, and the like.
Yuan-Ti, I think. Some sort of Zehir-like temple analog of the original ruby-eyed temple scene... except the adventurer's are dead, dying, or defeated and being sacrificed to a giant snake in the middle of the picture or something.
you have the right idea. We all know the MM is the monsters book. Their bid to take over the world. Why should they show players defeating them? That belongs in the players handbook. Death to the weak PCs!
A homage to the full page illo opposite the Lizard King entry in the 1E Fiend Folio
Or artwork depicting a dungeon run through Tucker's kobolds from the old Dragon editorial issue 127 I think.
But please put some interesting location around the monsters.