We've really seen our fair share of Dragons, Allagans and Voidsent over the years, and it surely wouldn't feel very fresh to base another expansion on it.
I guess the solution to that would be to primarily concentrate on the other aspects of Meracydia, like the peoples behind the Warring Triad. The worshippers of Sophia I find particularly interesting.
Of course there's a bunch of other stuff they could come up with.