Create an application for wagering with family and friends around sporting events where 2 people or a pool of people can wager with each other.
The smart contract system is perfect for creating an auto-executable decision following the outcome of a sporting event and automatic disbursement of funds. This idea could be applied to the simple idea of wagering against a single friend on the outcome of a basketball game to something as complex as a 100 person fantasy football pool. There would be no need for "a trusted person" to hold the money or any need for the large fees for wagering (10% or more) with a brick and mortar (or online) casino.
Small additions to normal gas fees and advertising could grow revenue.
Casino charge too much to wager. Most of the time we just want to wager with friends.