Or maybe a simple customizable option? As I am sure many would like a beanie or keychain, yet how many would buy them. Shirts are pretty common to use and wear while accessories are not that likely to buy in my opinion.
So adding a customizable option instead of those two things would lead to the popularity of certain items and would be added on to the shop for a price if popularity was good enough.
...
Yet still how would they be created and how would money be made for my idea or yours?