I think it would be great if the site allowed for the t-shirt maker to design both the front and back of a shirt, so that you get both sides of the same product.

Having an eraser in the T-shirt Maker, like Paint, to just erase certain parts of the design

Being able to upload my own fonts to the T-shirt Maker instead of just using the ones that are already there

Would be good if there's an option to upload our own background on t-shirt maker/mockup generator.