1> Trap information for traps of different shapes and levels
2> Hazard information for hazards of different shapes and levels
3> Organization information for how the different monsters might fit together in a larger group. I'd also like to find suggestions on Villianous Religious groups or Magical Cabals and how they work and operate
4> Lair information for how the monsters might manipulate their environment and use it as 'part of the encounter' instead repeatedly finding the monster sitting in a 'bare' room.
5> Several linked series of story seeds and general story seeds.
6> Sketched out details of plug and play encounters for the GM that has to assemble pieces in a hurry for players that went somewhere they did not plan or need to prepare an adventure in a hurry.
7> Guide lines for monster construction or adapting the provided monsters to new situations and encounters.
Ok, I am not so sure about the traps and hazards

but everything else in the list looks very nice. I'd be willing to accept 30-35% less monsters in the first MM to make space for this additional info.
On the top of my list would be points 4 and 6. I would like the presenation of each monster to be done thinking of the 3 "main phases" of the D&D game:
the plot, the exploration, the combat.
1) For the plot phase, which includes investigations and non-combat interactions, we need mostly fluff about how does a monster fit with the world, i.e. the ecology and eventually the society of this species, the archetypical characters. Some stats such as non-combat
skills can be useful too as a reference, if you can/want to have a verbal/social interaction with these creatures. I don't want too much fluff tho... no setting-specific stuff such as names. The DM can flesh out the exact names and addresses of the individual monsters, or use those provided by a campaign settings sourcebook.
2) For the exploration phase, it would be useful to have a list of features of a typical monster's lair (or building/house/castle in case it's a civilized creature) including treasures, and guidelines on
how to run the monster behaviour during the exploration phase, including relevant stats (e.g. perception and sneak abilities). Is this monster stationary, does it stalk and observe intruders, does it tend to hide or flee, does it hunt, does it see intruders as food, does it group with its kin or others, does it elaborate tactics with them to surprise intruders, does it kill or capture, does it setup traps...? All these info can provide improvement to the boring "party explores the dungeon, monster waits in its room".
3) For the combat phase, besides the obvious stats block, I want guidelines on the monster's tactics. Don't forget to summarize which of its abilities the monster is usually going to use predominantly, and in which case. Highlight which are its best weapons and its weakness points. Maybe write a small flowchart or checklist on what the monsters does in the most common events: when it is losing the battle, when the PCs are running away, when one PC is killed, when it is surrounded or attacked by magic or puzzled by invisible foes... can the monster be submitted, captured or even calmed down? It doesn't have to be a huge list, but it would be nice that each monster has at least some specific ideas tied to it: maybe this one creature is known to be more easily backed off if confronted with aggressivity and display of power, while another one may react the opposite and instead ignore foes that don't present an immediate threat.
edit: I'd be happy with a MM where every monster gets 2 pages (150 monsters is ok for me):
- 1/2 page for combat stats, guidelines and tactics
- 1/4 page for ecology/social stats and guidelines
- 1/4 page for exploration-phase stats and guidelines
- 1/2 page for the picture
- 1/2 page of additional stuff (e.g. sample characters with classes, enlarged/elite versions) or extra space for previous points if needed