A t-shirt printer using wash-off colors that require a special detergent to dissolve/remove. The colors are not affected by rain, sweat, etc.
Print anything you like on your blank shirts before you wear them that day. Start with a clean slate after every wash.
Your shirts are always unique.
This turns you into a walking daily tweet/meme.
Business idea (sell the colors + detergent + printer sets).
A home t-shirt printer using a few textile colors of your choice. The colors are dissolved with a detergent - purposefully created for this. They aren't affected by sweat or water and leave no trace on your other clothes. I'm not good with chemistry. Someone else will have to make this idea feasible. The colors should release from all surfaces when the detergent is present.
Find yourself some blank shirts with a style of your choice. Come up with lists of cool stuff to put on.
There are ready made printers that could be used for this purpose. They could be branded with your brand and sold to your customers in sets along with the colors, detergent, blank shirts.
All that's left to do is to create the colors and detergent combo.