I have been slaving for hours looking through the code for a way to give a new WorldType a unique WorldProvider without over-writing WorldProvider's getProviderForDimension. Hell, the closest I got was registering my new WorldProvider over the already created one, however some features like cloud height are set in stone by that point, rendering the entire thing moot (atleast to me). Also re-assigning World's worldProvider is impossible as it is final, and as far as I know is only assignable through World's constructor.
The point is, a nice feature for custom WorldTypes would be WorldProvider hooks that allow all the cool things only afforded in extra Dimensions.