Backstory: Personalization and customization is becoming more and more trendy like those dog socks, custom phone cases and pop-sockets.
The Idea: Custom printed cups.
Deep Dive: Similar to the the "dog socks," you would build a platform for customers to upload an image or select from stock patterns to print on a cup. With the improvement of digital printers and engravers, you could offer materials like acrylic and stainless steel.
Validation: I have three friends with those custom dog socks, I never thought those would be that popular/trendy. The key here is to nail it with the marketing.
Monetization: An ecommerce play with high margins. Nobody has hit it big in the space yet, being the first would afford you the option to charge a premium.