Definitely not a unique idea (see also: http://www.threadless.com), but I wonder if it could be bigger if it were more community oriented (Get your weekly r/pics or r/WTF shirts!), somewhere between the Shirt Derby (weekly, but with themes curated by Woot) and Teespring (highly specific, one time designs for a community and cause).
Depending on the size of the community and distribution of votes, it need not necessarily even come down to a single tee choice. Perhaps the top N tees.
