It is common for photographers to agonize over their next project. We want something original, fresh, interesting, challenging, publishable, yet accessible. There's no point in dreaming of a project on Hawaiian rain forests if you are in Pittsburgh and your travel budget can only get you to Ohio.
It might be worth considering just how important subject matter really is to our photography. If it's really important, then perhaps there are ways of approaching the choice of subject which are better or faster or more reliable or whatever, and if it turns out it isn't all that important, then why are we agonizing over it.
Let's start of with some basic facts.
1) there are no new subjects
2) there are no new approaches
3) there are no new techniques
Yes, I know, these are pretty bold statements but let me explain. With millions of serious photographers in the world the odds of you or I coming up with something new in any of the above categories is slim. Often we come across writings from more than 100 years ago agonizing over the same issues we struggle with today - and no matter how original someone is on subject matter, someone else is likely to have done it first.
You might be inclined to ask (quite reasonably), 'well, if that's the case, what's the point of even trying to photograph?"
There are a limited number of subjects or at least categories that we can photograph - people, landscape, machinery, buildings, nudes, still life, etc. On the other hand, there are an infinite number of ways of seeing something. When you look at an object, you don't see the same object that I do. You see it through eyes and with a brain which has completely different experiences, attitudes, feelings and values and each of these subtly and sometimes vastly affects how we photograph the subject.
If the only photographer you admire and collect is Ansel Adams, then it's natural to emulate him but even there, Adams isn't you and your images will be different. This can be a problem since you can easily get frustrated when your images don't turn out like Adam's. Sure, sometimes that's for technical reasons and just plain skill, but not always.
I have a number of images from my youth which even today are strong and hold up and yet I didn't give myself credit for them at the time.
As we learn about the work of many photographers, we can't help being influenced by all this other work. There could never be another Ansel Adams because the times have changed, there are too many newer photographers who influence us.
So, picking a subject because it's new isn't going to work and fortunately we will bring ourselves into our images of these "old" subjects.
On the other hand, surely some subjects work better than others? Sure, for the individual photographer. I like photographing old industrial sites, someone else might find it nigh impossible to come up with a decent image at the sites where I revel in it.
Clearly some subjects and more particularly, locations, provide more opportunity to make images than others. They have more parts that are interesting, a better selection of viewpoints that are good, better and clearer line of sight, greater textures, more interesting shapes, shadows, lines and whatnot.
Some setups have one fatal flaw, which cannot be changed or outwaited and you simply have to move on.
Of course, the amount of available material is pretty much independent of choice of subject category and a lot to do with the specific subject or location. When photographing architecture it's a lot easier to work with a building with interesting shapes, surfaces that reflect light in interesting ways, and which is accessible - ie. not jambed up tight to parking garages on either side and immediately across the road.
You might decide to photograph glassware.It's going to be a lot easier if the glassware is interesting - in shape, tone, reflections, colour etc. Location will be important - whether it's in your kitchen cupboard or against a mirror or next to a window.
The trick then isn't in selecting glassware, it's in finding the right glassware in the best location.
This would suggest that just about any subject would do, if you have at least a passing interest in it. Where you have to spend the time and use your initiative is in selecting the right example of that subject in the correct location and under the best circumstances.
This may seem pretty obvious, but I suspect that many photographers spend an inordinate amount of time agonizing over the first part and paying little attention to the next two - to the detriment of their images.