The game I'm making will be based on the FF book "Five Go off to Camp".
It features a "spook train" story, where the Five must unravel the mystery of the ghost trains they hear while on a camping trip. As well as having some interesting locations to design, such as an underground tunnel system built under a set of hills, as well as a farmhouse and some scenic fields by a river.
The main gameplay will involve two main mechanics. The first will be rescuing Timmy after he falls down a hole, where you will play as George. The second will involve escaping from an enclosed tunnel before the bad guys get you. Sweet.