To be honest, if they were looking for a way to do a modular MM I am pretty sure they would be thinking digital. Digital lets you decide which monsters you want, it is obviously a very compact format with excellent searchability, and as a bonus it lets you edit the entries with a minimum of fuss. You could them print out your monster sheets for use in the game later, or just use the digital monster manual as-is with your laptop, smartphone, tablet, or machine-to-mind interface.
I would like the Monster Manual to include a good hefty helping of iconic monsters, with multiple entries for a single monster type in most cases. I'd also like them to include a few new monsters, carefully chosen to fill gaps in their creature lineup so that they are not superfluous, so that we have some things we've never seen before to ponder as we chew through all the old, comfortable, nostalgic D&D baddies.
I want a chapter devoted to monster creation, both the rules for how to calculate their stats as well as a discussion of how to make monsters that that are interesting in combat and in context of your game world.
I'd like a chapter on designing encounters, with a discussion of monster roles (a la 4E) and how to insert terrain, hazards, and interactive map features in a fight. Since "boss" encounters of some kind make an appearance in most D&D games, I'd like to see a page or two devoted to discussing just that.
I know that others have mentioned the above features, and that some feel they more properly belong in the DMG. Organizationally it makes sense to me to include all the monster stuff in one place.
And that, I think, would be my ideal MM!