Two forms of dance/fitness that I think will become huge:
Capoiera. This intriguing martial arts has been around a long time, especially at the hippy camps, but will go mainstream with Halle Berry's use of the moves in Catwoman.
Bhangra. The Indian dance style most commonly used in Bollywood dances, is going to hit the bigtime with a Bend It Like Beckham style movie behind it, (as well as The Guru from 2002), and a resurgence in the gyms of NYC.