I think Harris is spot on about considering what you want to teach rather than just trends.
Here are a few general things to think about: A general group exercise certification costs money to get, and to maintain. Everything you add on top of it also costs to get and to maintain. I can remember taking things decades ago that were ‘going to be the next big thing’ and have really just disappeared. Before you pay a lot of money for a certification in something you want to know that you will get back the cost of the training. How much does the cert. cost to get, how much to renew, how much to pay for CECs. What is the average pay rate to teach this class in your market? Do you own your own business? If so poll your students“. would they be interested in your adding these classes. How many other businesses are there in your area and how many offer this type of class? If there are lots of places offering it and you work at studios you may have more opportunity for work. If you own your own it may be harder to bring new people in to your places. On the other hand if there are few you need to figure out whether that is because no one has exploited this niche, or because not many people are interested in it.
Generally students love to try new stuff. For many students whether they stay with it has as much to do with the skills and professionalism of the instructor as with the format (from what I have seen), that does not mean a strength trainer is going to start doing chair yoga, but a person after a strength workout will often follow a teacher with great teaching skills who helps them feel the burn whether they call the class one name or another.
That said with some things (like zumba) there can be a strong following for the format.
I am just saying before you spend a ton on training remember how many people did step aerobics 20 years ago. And ask yourself what happened to the money people spent on workshops to learn jump a ropics or slide aerobics.
And as a yoga teacher I would say: even if yoga is popular in your area, and Yes, you can earn a good per class fee, it is probably not the best add on training to get. The training is expensive, and to really work in a yoga studio you are likely to be asked to have a 200 hour certification at a minimum. Senior teachers tend to have many years of intensive training.