I work for a startup footwear company in Portland http://ryzwear.com/ that has a similar model to Threadless- we hold monthly footwear design competitions and produce the winner's design, splitting the profits with the artist. I invite you to visit our site and post designs and/or give feedback about our concept.
I definitely think that is a possibility. The great thing about a startup is that we can adapt quickly to other opportunities. For now, we are introducing a few different templates to design on (high and low top, slip-on). Thanks for the feedback!