I think you're looking at this the wrong way. Your suggesting building a competitor for Teespring.com and focusing the features that Teespring doesn't have (or limits) with the assumption that those features are what is stopping Teespring's growth. What you should rather focus on is figuring out what is the most frustrating part of using a service like Teespring as seen by existing users and see if solving that problem has value. Building a pure copy with some added features is probably not a good idea.