I think NSMB to NSMB Wii will more like Mario Kart DS to Mario Kart Wii.
Animal Crossing never has been a big franchise on consoles to begin with, whereas Mario always has been, and NSMB Wii on the Wii has all new levels (like Mario Kart Wii has new courses), and the addition of a major new feature -- multiplayer, like MK Wii brought the easy accessibility of the Wii Wheel to the table.
In terms of the game experience, I think the addition of the 4-player multi is actually bigger than adding the Wii Wheel to Mario Kart or anything they added to Animal Crossing. It will be a big deal and it changes the game a lot. Playing multiplayer Little Big Planet to me is a huge difference from just the 1-player game, I have no doubt it will be the same deal for Mario.
NSMB like Mario Kart Wii I think is going to be the perfect "bridge" game ... a game core gamers and casuals can enjoy together, much like Mario Kart Wii.
$199.99 Wii + New Super Mario Bros. Wii = massive sales.
