Some of the classes that will work for you will depend on your demographics. Zumba, TRX, bootcamp, HIIT classes, Insanity, spinning, and yoga are all going strong where I live. Mixed classes such as cardio intervals and strength training are also popular and fun. You could also look into teaching a ViPR class–which is strength training blended with functional movement patterns using a weighted tube. There are many classes to choose from, so it can be overwhelming.
You may want to check out what local gyms and community centers in your area are offering. In terms of what you teach, it should also be something that you have a desire or passion to teach!
Good luck to you!