I did like what Hive account@kieranstone said below but I also think nitches are more than just what type of photography. I think it is also about branding yourself and what sets you apart from other photographers. Maybe it is about how you use color or light, or perhaps you shoot from a unique perspective. Really define what your style is. Then research different markets to sell your photography and pick the best ones for your style and type and what you want overall as an artist.