Can't wait for that Colossal Animated House and Elder Black Pudding!
Seriously, Purple Worm tops my list (make it just like the MM pic). Tarrasque is a given but I don't know for sure if I'd pick one up. I hope they don't do any dinosaurs--I can get those at the toy stores if I ever need one. Vermin would be cool. While the Roc fits the bill, I can't really see them being popular enough to warrant a figure.
There's quite a few massive monsters out there, but not a lot that really stand out. I think most of the more interesting monsters are in the large to huge range. There also are few (if any) FR monsters in this size range. Dracolich being the only possibility I can think of.
MM2: Bone Ooze (G), Cloud Ray (C), Corpse Gatherer (G), Dire Elephant (G), Fiendwurm (G), Flesh Jelly (G), Mountain Giant (C), Leviathan (C), Dread Linnorm (C), Corpse Tearer Linnorm (G), Megalodon (G), Megapede (C), Orcwort (C), Tempest (G), Teratomorph (G)
Fiend Folio: Blackstone Gigant (G), Century Worm (G), Lucent Worm (G), Sea Drake (G), Thunder Worm (C), Ulgurstasta (G)
MM3: Necronaut (G), Omnimental (G), Plague Brush (C), Siege Crab (G,C)
MotP: Astral Dreadnaught (G)
Frostburn: Shivad (G), Zeuglodon (G)
Sandstorm: Crawling Apocalypse (G)
Planar Handbook: Astral Kraken (G), Elsewhale (G)
Of all these, I can only see, perhaps, the Corpse Gatherer, Fiendwurm, Blackstone Gigant, Ulgurstasta, and the Astral Dreadnaught as likely contenders. Maybe a Linnorm too, but doubtful. If the size range were extended to Huge as well, there would be several dozen more interesting choices.
BTW: Stock Hellfire Wyrm is Huge, not Gargantuan, as is the Titan.