ACSM just released their worldwide trend survey which may help guide you. (http://journals.lww.com/acsm-healthfitness/Fulltext/2013/11000/Now_Trend…)
The three top trends according to the ACSM survey are: 1) High intensity interval training, 2) Bodyweight training and 3) Increased fitness trainer professionalism. There appears to be a high incidence of injury in high intensity interval training, which makes me wonder if it’s going to maintain its top position. A very fast rise with a serious shortcoming is something that has all the makings of a fad, in my opinion.
It’s interesting that the third top trend is about professionalism; clients are demanding that their trainers have the highest possible levels of training. This is a trend that I’ve been watching for some time. There’s been a definite need for professional certification of trainers, along with an expansion of the many different courses and degrees that confer the appropriate skills.
Although, Zumba is not on that list, I concur with Christine. You should definitely conduct your own local survey to see what is trending near you and of course teach what you are passionate about!
I hope this helps, and wish you the best in your search!
Best in Health,