One way to create outdoor stock imagery that might be in better demand is to try some self assignments. Try acting like a photo buyer looking for an image and give yourself the assignment.
In many areas of stock photography, clients are looking for pretty pictures for their calendars and cards, but in more commercial markets clients look to buy images that have concepts like Teamwork, Perseverance, Challenge, Longevity, and so on.
It is a little more challenging to apply concepts to nature photography but think about what the Bald Eagle represents: America, strength, sharpness, patriotism, and more. I also shoot people outdoors and find the application of concepts much easier, but the demand for the same in nature is there from time to time.
Keeping this in mind when you travel and photograph you will shoot as you normally do, but simply keeping an eye out for subjects and details that fit concepts can create images that might be more in demand.