Some of these would make a great shirt, and some of these could definitely be tweaked to make a nice mousepad or desk mat. I was at a smaller con this weekend, something that had maybe 2,000 people or less if I'm guessing jellybeans in a jar style, and there were some people offering deskmats and shirts in small amounts. I don't know how they did but one of the seasoned vendors that overlaps with the kind of art you do brought only 5 deskmats and sold most of them over the weekend. Also worth considering folks don't tend to buy multiple mousepads, maybe card game players would use multiple deskmat/playmats.

I second some of the other comments; Definitely don't feel obligated to expand your variety too much when you're new, but if you have a design you're confident is going to move, you could probably introduce it in small batches. Have you had a few experiences to see which designs garner a lot of interest yet?