It kinda has to be a Cave area next. If you look at the unreleased DW Pokemon, it's mostly just the Dragon, Rock, Ground, Ice, & Dark-types all of which fit nicely in a cave.
Notable cave-dwelling Dream World Pokemon off the top of my head:
Gible
Bagon
Swinub
Sandshrew
Hippopotas
Sableye
Spiritomb
Larvitar
Snorunt
Onix
Cranidos
Good Thick Fat Mamo, Rough Skin Chomp (if he gets a second chance), Sand Power Hippowdon, and Bulkier-Excadrill (Sandslash) should all come in at the same time. Pro-Sand-ban will love that. (Unless they spilt it and make a Snow area or Desert area or some BS like that)
They're CLEARLY saving the best area for last. :/
I actually had a REALLY similar theory to this, only it was a desert area instead of a cave, and it included stuff like Cacnea, Sandshrew, Shieldon, Trapinch, Numel etc. But yeah, all the other stuff will be included in future areas, no doubt.
Also, I hate to string off topic a little, but how the heck are you getting so many points so quickly? I only have 590 and I want to unlock the other areas :(