You could look through the Standard Industrial Classification (SIC) codes. That will give an overview of what exists.

Fair warning, though: If you see, say, "Rubber and plastic footwear" (code 3021), and you think "Hey, there ought to be some low-hanging fruit there - how hard could it be?", well, you will probably find that the state of the art there is considerably more sophisticated than you thought.

That is: I suspect that picking a category, and deciding that there should be some opportunity there, is a poor way to proceed.