For me it has to be something that is I’m familiar with. So I don’t have much time to learn the ABC of things. Then I consider the market relative to competition. Even if the market is small but as long as it’s no dying it’s worth a look for me. Smaller market is easier to enter as there are relatively very few competitors. Slicing the pie won’t be a problem then.
However knowing how to promote will be the big concern here.