it is easy to photograph things in your comfort zone.
when you start doing things that are difficult for you as a person
that is how you can grow as a photographer and as a person.
who knows, maybe you will find a project in something that terrifies you ...
I'm reading (perhaps mis-reading) a slightly different question here. I think (and the OP can correct me if I'm wrong) that it's not a question of "should I step outside my comfort zone" but rather one of randomness vs. deliberateness - "found" images that break from what you were intending to take at the time vs. sticking to your intentions and not taking the "found" image. I agree with John that it's a good exercise for creative growth to sit down and say, "I always photograph trees... I'll push my boundaries and try photographing people/neon lights/buildings/cars/still life". But there's deliberateness in photographing that way, training the eye to look for and see the new subject. There can be benefit to just walking around with a camera to take pictures of whatever, as a form of visual note-taking, but I think it's harder to develop order out of chaos - if you're shooting whatever, you end up with a lot of individual images that don't fit anywhere. Yes, you might catch a few that would make good stock photos and could sell. But having a portfolio of catch-all will make it harder in the long run to sell the stuff that IS your style because people won't look for you and your work. They won't remember you, and you'll have to rely on Google searches delivering your image high enough in the results that buyers will get to it. Not really a good plan in my book.