Out of interest, I just did a quick (and unchecked, so excuse any errors) comparison of creatures that have appeared in the core monster books for 1st, 2nd and 3rd edition (specifically the 1ed MM, the 2ed Monstrous Compendiums (Vol 1 and 2) and the 3.5 MM) but which were not in the 4ed MM. Here's what I got:
Ankheg
Baboon
Badger
Bear, Black
Bear, Brown
Beetle, Giant Bombardier
Beetle, Giant Stag
Bison
Black Pudding
Blink Dog
Boar
Camel
Centaur
Centipede, Giant
Cockatrice
Couatl
Djinni
Dragon Turtle
Dragon, Brass
Dragon, Bronze
Dragon, Copper
Dragon, Gold
Dragon, Silver
Dragonne
Eagle, Giant
Elemental, Air
Elemental, Earth
Elemental, Fire
Elemental, Water
Elephant
Ghast
Giant, Cloud
Giant, Frost
Giant, Stone
Golem, Clay
Golem, Iron
Gray Ooze
Hag, Sea
Invisible Stalker
Lammasu
Leopard
Lion
Locathah
Lycanthrope, Werebear
Lycanthrope, Wereboar
Lycanthrope, Weretiger
Manta Ray
Merfolk
Mimic
Mule
Naga, Spirit
Naga, Water
Nixie
Nymph
Octopus, Giant
Owl
Pegasus
Pixie
Pony
Quasit
Remorhaz
Rust Monster
Shadow
Shark
Shrieker
Squid, Giant
Tiger
Triton
Violet Fungus
Whale
Will-o'-Wisp
Winter Wolf
Wolverine
A lot of these are animals, and I'm sceptical about seeing too many of those return. I also don't think we're likely to see the vanilla elementals any time soon in 4ed. But there are a lot of other classic creatures on that list that seem very likely candidates for the MM2.