Dixie Kong is a lot more likely than many people think; one of the most underrated candidates for the games.
Like Shortie said, Dixie's main advantage is that she was planned for Brawl, which shows that Sakurai has shown interest in her inclusion already. The reason why she was dropped from the Diddy Kong team was because of complications surrounding the duo, not because Sakurai felt she was unworthy of a spot. While Diddy Kong will forever remain solo, it doesn't mean that Dixie missed her chance as she can be implemented as her own character.
She also has the advantage of being a popular choice in Japan. Dixie gets brought up regularly there and Sakurai will notice the demand for her there. Her fan base will also receive a large boost if King K. Rool is confirmed to be in the game.
While King K. Rool is still the more likely newcomer, a character that has been planned for Brawl and enjoys popularity is not an unlikely addition. I think if any franchise receive a double addition, it will be Donkey Kong.