I would very much like to see a decent-length section on traps and environmental hazards... but whether it fits in the MM or the DMG is an open question.
For 5e, I would be inclined to put together a fairly short set of 'core' monsters - the staples of intelligent humanoids, animals (normal, giant, dire, swarms, and the like), and other classics (gargoyles, dragons).
And then a big emphasis on creating unique monsters for "guest starring" in your adventures - provide a big toolkit of powers, provide the mathematical framework for putting them together. And then finish with a decent number of examples.
I don't think the game needs 500+ types of monsters in the first MM. I'm not sure it even needs that many monsters at all. But it probably needs a lot of unique (or easily customised) forms of construct, undead, individual dragons, and the like.
Because that seems to be the way it works in the fiction - in Lord of the Rings you have the staple Orcs and Trolls, and then you have Shelob, the Nazgul, the Balrog, and other unique (or close to unique) creatures. Likewise, Conan seemed to fight a lot of men (civilised and savage), animals and the like, and then in most stories he fought one unique monster - a demon or 'god' or whatever. Likewise Elric, and Fafhrd and the Grey Mouser, and the like.