Traps and terrain.
There's enough for combat pillar, we need more exploration pillar.
The better question for me is, "what, if they put it in a Monster Manual 2, would actually prompt you to buy the book?" And that's a tough one. Frankly, nothing comes to mind.
Ribbing aside... I can't think of what monsters I would actually want to see in a Monster Manual 2. Most of the monster on my mind right now all belong in a book titled Creature Collection because they are Scarred Lands monsters, not general-use monsters.
"You fools, this isn't even my final form!!!"I'd like to see some monsters that can reasonably be encountered at all levels. Yes yes bounded accuracy blah blah...that's not what I mean. I mean a sort of monster that you could reasonably run into multiple times and still remain interesting across the levels.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.