I like the special world in New Super Mario Bros. Wii, but I think it should be open a different way. I mean I'm kind of sick of collecting star coins. I mean they can unlock other cool things. I know it's meant to be difficult, but finding all the star coins just to unlock special world just isn't fun, imo. It's long and drawn out so if you miss one and it's not in an obvious place, you spend forever trying to find one of them.
My favourite special world is Super Mario world's special world.
Here's what I think they should do.
New Super Mario Bros. 2
Each world has a special stage unlocked by getting all the star coins in that world. (Like DKCR)
When you get all the star coins in all the special stages, you get a special world like NSMBW
New Super Mario Bros. U
Basically like Super Mario world, there's a Star Road level, and at the end of it is another special world with a bunch of weird names levels