Backstory: The greeting card market is dominated by hallmark. Similar to how other "big business" markets can get disrupted by smaller players, this market is ripe for someone to offer a cheaper and more unique solution.
The Idea: Homemade greeting cards.
Deep Dive: Similar to how creatives sell on etsy, you would build a platform for designers to create and upload their greeting cards. Every time one is printed, they get a small portion of the fee. You can run contests similar to woot where users vote on the best card and that gets offered at a discount.
Validation: There are already some people selling greeting cards on etsy.
Monetization: You would make whatever profit is left over after shipping, cost of goods and designer fees.