I'm probably in the minority here, but I always thought Grunty's Lair was a bit confusing to navigate. All those doorways and areas made it maze-like, and it was complicated finding the area where you unlocked the world as well as the world itself, and they would usually be in completely opposite areas in the hub.
I would like it if the hub world was a bit more distinct and had more character to it that went with the theme of whatever world you would be entering in that area. Also, there's no rule that the hub world has to take place in some cursed, underground area. We don't even know the story of this game, so the hub world depends a bit on the circumstances we're thrust into.
Personally, I wouldn't mind if it was a bit more of an open area, or perhaps it starts closed off and steadily gets into more open areas as we went along, perhaps signifying our progression to the final boss fight (kind of like in Portal 2). Each area would have its own theme, so you wouldn't be confused over where each world was located.