Reggie has recently discussed that Nintendo is working on DLC for 3DS, including possibly for this game. That means that characters that didn't make the cut, such as Waluigi, Diddy, and Baby Mario, could eventually show up.
I hope that it does happen, but they make it worthwhile. I hope they add characters and maps, especially for battle mode. Maybe they can add character/map packs that will cost about $4 US each? That would be reasonable if they added about 2 or 3 characters and 2 maps per DLC pack; there could be Metroid themed levels (designed by Retro) and maybe even some cool levels based on Pikmin...that would be awesome!
Actually, on December 8th, there will be a major Firmware Update for the 3DS that will allow allow many new features, most notably 3D Video & many other functions will be added & yes, DLC will become available to all games, not just Mario Kart 7. So long story short? The answer is an absolute YES.