This.

There is currently only one MMORPG bitcoin gambling site: Dragon's Tale, http://www.dragons.tl/ while there are already many good bitcoin casinos, dice sites, poker sites and sportsbooks out there.

Maybe make something similar to Dragon's Tale, an MMO where players can gamble bitcoins on different kinds of fun games - both luck based and skill based, solo and PVP, while also interacting with each other socially.

The market is relatively saturated with all the traditional mainstream types of gambling. But there is only one bitcoin mmo gambling site. So while not unique, it is a relatively under-served market.