If you have automatic charges as a merchant (monthly fees you charge your customers), sometimes they are declined. Many of these could be circumvented.
A mechanism that allows you to check a charge in advance without actually charging the card. If you get a reply that the charge will not go through, you can alert the customer before the actual charge is applied to his card. It’s better for the client and you.
For card lovers, manufacturers, historians, and collectors.
A website cataloging all playing card designs know to man. From the distant past to the latest ones. People can also submit their designs and manufacturers can find new designs among these.
For kids: help them have some structure to their story, and show them many different possibilities. For all: help them get going with their imagination, and have fun.
Have different card packs for different aspects of storytelling. Structure, genres (romantic, detective, fantasy etc.), time periods, style (narrated, first person etc.), atmosphere etc. Can also be done online (website, app), and an online community where the finished stories can be published.