No, because the entire roster has officially been revealed in the latest Nintendo Direct. It is impossible for him to be added. Nintendo wouldn't lie about having the completed roster, and then turn around and put King Boo as an additional character.
Shouldn't this be merged with the Mario Kart 8 topic?
Anyways yeah King Boo is cool I guess. It'd be really nice if they'd use his Luigi's Mansion design more often. He looks way more badass that way.
It doesn't appear that he'll be returning for Mario Kart 8 though. I think it's pretty silly that he was turned down for the likes of Pink Gold Peach and Baby Rosalina.
King Boo was my main second character in Mario Kart Double Dash (the first being Yoshi), and I still played as him now and then in Mario Kart Wii. I like him. Kinda sad to see that he, Bowser Jr. and Dry Bones won't appear in Mario Kart 8.