What environments do you want? How big do you want it to be? Progression-wise, how do you want things to go?
I thought Unova was a bit too circular, but B/W2 fixed most of the problems I had with the Unova region. Other than that, I don't care if the game itself is linear, just make us feel as if we're traveling far and wide across the region in all directions. --- This is the internet, all opinions are considered invalid.