How did you find your nitch?
But more importantly. In a world with only so many possibilities, how do you keep it?
At no time in the history of the world has photography been in the hands of so many. No matter.
Broadly speaking, two approaches work well:-
1. Common subject, done to death by others, now photographed in a completely new and different way. Alex M is very good at this - the clownfish/fisheye/teleconverter is a good example.
2. Photographing the unknown or rarely done. Finding the unknown is challenging - sometimes you just don't know what you don't know (no pun intended!) - but there is an entire planet out there just begging for a fresh pair of eyes.
There is a whiteboard on my office wall, covered with ideas, concepts and theories that are all worthy of coverage. Not all (very few) have been pitched at editors but thats only because I don't have the time to do everything and need to concentrate on those where the risk/reward combination is in my favour. At the rate I'm going, there is 10 years of work on that board. Some time-sensitive ideas are planned years from now, but I keep them all fresh in my mind.
Dean is right. There is a lot of potential in crap vis.......its not an easy place to work, but as an example, consider this image:-

Taken in HMNB Clyde, home of the UK nuclear submarine fleet, RN divers fix submarines below the waterline and between the inner pressure hull and outer skin. These chaps are walking across the seabed, going to work. The vis is less than ideal.....but the image tells a story. Security issues aside, few would normally bother diving in the harbour and even less would bother carrying a camera but places like this offer much potential.
Good luck!