My next thought were about quotes. I have seen quotes and sayings on shirts and thought the idea of famous quotes seems like a great way to tap in to a pre-installed audience. Obviously we would all like this, and usually have to pay for the privilege.
This also made me wonder about memes. I realize a meme pic isn't necessarily a great shirt design, but we have all seen pop-culture blurbs on t-shirts.
Well, again, I found that this isn't a great way to get a installed audience. This very detailed post explained that copyright law actually protects quotes. A person can quote someone in conversation and reference the original speaker, but one can not print the saying on a shirt.
So, how does one get permission to use a quote? Another great article describes the process of doing just that. https://www.copyediting.com/how-to-get-permission-to-use-quotes-and-graphics/#.WPvNZ4WcGe0 .
While a lawyer will be needed to actually execute such a contract, at least now, we know what is possible and how to do it.