The model thus far has been based on expanding Expedition locations. If this is continued, we'll probably see an Amazon Horror and a Secrets of the Himalayas which links to Tunguska.
Alternatively expansions could be based on continents, in which case Europe has to be high up on the list, preferably with some kind of Orient Express option. In this argument Australia also seems likely due to the story The shadow out of time. A boat route from Port Hedland (Western coats of Australia) to Cape Town really ought to exist, given the significance of that location in the Masks of Nyarlathotep.
I can't imagine an Arkham-based expansion - although many of the stories are set there, you've already got Arkham Horror to cover that ground. New York, a nascent Los Angeles or even some kind of Bradbury inspired mid-America Green Town would interest me more.
One thing I would like to see is an expanded board which contained all the sideboards in one place. In EH, your main enemy is time - you've got a limited amount of time, so if you want to go and explore Stonehenge or the Nile instead of going to the next clue or closing that gate, then go right ahead. More options is a good thing.
As for the Dreamlands, it'll be a separate game if it happens. Personally I'd recommend the Arkham Nightmare for Arkham Horror in the intervening time.
You could also make a case for a decent Hyboria co-op game here. The REH stories are nominally linked in with the HPL ones - the authors certainly wrote for the same audience.