For starters, I'd call it "Fiend Folio 2" rather than MMV. Beyond the fact that the higher the sequel number, the higher the expected "suckitude", the MM name has become too muddled at this point to give you an idea of what to expect. Go back to the last great monster collection and pick up the pieces from there.
I'd like to see a 50-50 split of updated creatures from past editions and "new" creatures. The new creatures should include some of the best critters from Dragon, Dungeon, and modules, similar to what the Fiend Folio did. Just a few great beasties that deserve the attention: rotripper and kurge (Dra Annual 5), the numerous new demons and other creatures in the Demonomicon articles (I'm not talking the archfiends themselves), just about anything from the last Creature Catalog, the Kyuss-associated creatures (I hesitate to use the term "wormspawn" for fear of assoication with dragonspawn), the spirotovore energons from Bastion of Broken Souls, etc. Of the classics, finish rounding out the celestials, rilmani, and yugoloths. Bring back fan-favorites like the quickling, rakasta, sheens, ferrous dragons, ghazneths, etc.
I'd like to see them build upon existing groups of creatures such as nerra, ethergaunts, rilmani, rather than creating new, less interesting batches (I'm looking at you mindshredders).
How about an even spread of creatures across CRs 1-20 for a change, plus a few epic creatures?
Two words: More templates!
I want to see almost no humanoids, less monstrous humanoids and undead, more animals, fey, giants, and vermins, and about the same amount of the other creature types.
Absolutely, positively no MM creatures with class levels!
I'd like to see appendices better detailing how to advance creatures, and how to do it quickly, so people will feel more comfortable with the process and stop demanding things like the MM monsters with class levels. I'd like some of the more common abilities like sneak attack, etc. to move to the glossary, rather than be reprinted in individual entries (they've taken steps in this direction already).
I'll echo Demiurge's suggestion to follow the Dungeon stat block format, for all the same reasons.
Cut back on the "In Eberron/In the Realms" sections unless they actually have something worthwhile to say. If the creature is usually found in deserts, I don't need a suggestion to place the creature in the Raurin desert or the Anauroch...that seems to go without saying.
More critters for the less populated planes: Pandemonium, Positive and Negative Energy planes, Far Realm, Temporal Energy Plane, etc. (I realize some of those are variant planes, but they've used 'em in the past).
Make some truly gigantic giants (Gargantuan and Colossal).
Break new ground. The Fiend Folio introduced the swarm subtype, grafts, and symbionts, all game-changing elements that continue to pop up in later supplements.
Keep the expanded ecology information from the more recent books, as more flavor is never a bad thing. Keep the lore, but try to make it more useful. My players are much more interested in knowing if a critter is vulnerable to a certain energy type or a certain material is needed to bypass damage reduction than whether the creature is used as a mount by other creatures.