About a month ago we were introduced to a new playable character, Rosalina, during the Nintendo Direct that showed off what appeared to be a Rainbow Road stage in the cutscene for Super Smash Bros. This was later unveiled to be a Nintendo 3DS exclusive stage based off of the handheld Mario Kart Rainbow Road.
Pic of the day. Watch out for the Shy Guys driving down Rainbow Road.
This sounds similar to some of the past racing stages in the series, but with the stage being cruise based as Sakurai already said, it should have a lot of potential to be a ton of fun. Tomorrow is the last update of the week, so hopefully we will get something big to end the week.