I sincerely hope they go for the "core" monsters and cut the fat. MM1 is full of things you're not likely to need stats for, really. As my MM1 is hiding, I'll use the SRD for examples (which means I may name some not really in the MM1 so shhh):
Should go (can come back in later MMs or stay away):
Achaierai
Aranea
Arrowhawk
Athach
Azer (but they won't)
Belker
Chaos Beast
Chuul
Couatl
Darkmantle
Delver
Destrachan
Devourer
Digester
Dinosaurs (all)
Dire Animals (most)
Dragonne
Ethereal Filcher
Ethereal Marauder
Formians (all)
Gray Render
Howler
Kraken
Krenshar
Lammasu
Lillend
All non-werewolf lycanthropes
Magmin
Merfolk
Mohrg
Phantom Fungus (man if this thing is in 4E MM I will ritually throw the book across the room)
Phasm
Rast
Ravid
Scorpionfolk
Sea Cat
Shocker Lizard
Spider Eater
Tendriculos
Tojanida
Vargouille
Winter Wolf
Yrthak
It's not that these monsters suck, though many of them do. It's that they're either wierd or often unused (and I'm sure ENworld can produce anecdotes about all of them, but come on...), and basically represent a HUGE amount of fat that can be trimmed.
NEED to keep
Animals (most)
Vermin (most)
Swarms (some)
Need to gain
Pre-gen stats for human and demihuman bandits, pirates, etc.