Your own label.
Your stuff that counts, Spreadshirt is helping you to realize your ideas but in the end its all about your design. It is your personal thing.
Let the people out there see your logo as a invitation. A invitation to be their own label.
Explanation of shape, colour, usage and additional images in the full post.
Consisting of positive and negative space, it shows the viewer that the shirt is somehow empty, waiting to be filled by the customer.
It is literally a invitation to the viewer to realize his ideas - Spreadshirt is giving the frame (in the logo as well as the business), possibility and support, the rest is up to you.
The logo is simple and reduced. This appears generous and serious, which is confirmed by the confident square-element. A little tweak to modernity is the asterisk: On the one hand it is telling the viewer “but wait, there IS a little more about it, check it out!” on the other hand it is leading to your new tag line - as a kind of footnote - that fits nicely into the invitation-idea.
The logo color should not be exciting, the content ( background) should be exciting: The users idea and design is what gives content, color and spice, see below (the flower-pattern is for position only)
Print material could reach a great effect if the logo was blanked out.
A hang tag attached to the shirt could have several pages, the second page is showing a recent design (maybe a monthly contest among users) or any pattern. This shines through the cut logo.
A business card or flyer as a single sided element works by itself: While the viewer is holding the card he sees the environment through the blanked out logo and is already encouraged to think about new designs. He is now his very own label, he can do every design he wants, just look through the shirt and see what ideas will appear.
Online material could use randomly changing images of current user designs or any similar pattern.