pattern recognition is your friend (as the Good Book says, eyes to see and ears to hear)
step back and really look with an emotionally detached mind
don't look at what those others shops offered, look at what they did not offer
just as people go along to get along (hive mind), they naturally have an innate deep desire to be free individuals
that is a stream you may wish to drop your tube into and see where it takes you
here is a couple things to consider:
~ learn to airbrush (each tee is a singular artwork + the paints are american-made non-toxic, two big selling features)
~ peoples photos turned into artworks for tee's, framed prints, etc. (what do people like? their cars, their pets, their children/grandchildren, their vacation memories, etc.)
both of the above also allows for the customer to have creative input if they desire, making them feel part of the process and finished product
( if you want to see some examples pm me your email)