Man, both the commercials I posted earlier tonight are turning out to be treasure troves of info! Thanks to Jumpman 64, we now can catch a glimpse of a possible 5 as of yet unannounced characters. Check out the screens below to see the goods.
In the image below you can see ShyGuy in the back left, a Koopa Troopa next to Mario, and Boo next to Peach. Jumpman 64 points out that Boo might be an item/powerup.
In this screen capture, you can see Dixie Kong to the right of Sabotender. She has her back turned towards the screen. On the left hand side of Sabotender you can see some sort of round, yellow character. Once again, this character may be an item/powerup.
*UPDATE* Birdo! How could I forget Birdo. I meant to put up this picture this morning, but I guess my brain was shot since it was 7! Thanks to Insanedoodle58 for the heads up.