I'd like more involvement with the map, too, along with more freedom of the enemies placed on the map, as well as a larger variety of rewards. To be honest, I would have preferred a world or two less, (I found some aspects of World 1, 5, and 6 generic enough they could have been used anywhere,) with more stages in each, in order to foster a more complicated world map, closer to SMB 3's. A lot of ideas utilized in SMW were honestly scaled back, such as how the landscape developed according to your progression.
The Bullet Bill type of encounters were my favorite, with Piranha Plants being the least favorite, I think, but I think better things could have been done, such as enemies moving from stage to stage, or just moving like Hammer Bros. did back in SMB3. There's a lot of options overlooked that would've been interesting, is all I'm saying, and longer worlds would allow more "space" to flesh them out, making each world more unique.