Devil Swine, Wearboar...I'm sure if they can make Vine Horrors out of Trolls, and Mudmen out of Clay Golems, they can manage to make one out of the other.
On that note, I wonder what SSG might be able to reskin the dinosaurs into someday? Drakes? There's a whole lot of Khorvaire that's just full of 'em that'd be great to see, too.
As for classic monsters I love and want to see added — I was really hoping to see a Kraken show up in Saltmarsh. Like Aelonwy said, there's a ton of undersea monsters left to see, I'll throw Marids on the pile too.
Flail Snail, Roper, and a bunch of the really ugly fiends like Manes, Lemures, Bone Devils (I can wear one's face, but where is the rest of him?), Ice Devils and Nalfeshnee are some of my favourites. I think the animation work required to do most of them justice means it'll be a long time if ever before I see Ol' Flaily in DDO.
Couatls are kind of a big deal in Eberron lore, it'd be cool to see one show up as an NPC.