I have my students create a 'mini design document' including the following:

- Overall Vision of game (this is kind of the brain dump you refer to)

- Storyline - 2 - 3 paragraph storyline written as a story (not as a description of what the game will be like)

- Characters - list of playable and nonplayable charachters including enemies, bosses, npcs, etc. I require a description of each character including the attributes of the character

- Items - list and descriptions of all items in game (obstacles, keys, power ups, coins, etc.)

- level descriptions - level by level description of the game level environment, what is present in each level, etc.

- scoring mechanisms - description of scoring (lives, health, score, currency, etc.)

- winning / losing scenario - description of what is required to win the game (i.e. defeat the final boss and save the princess) or lose the game (lose 3 lives, etc.)

-game controls - how will the player be controlled in the game (arrow keys, joystick, mouse, etc.)

Hope that helps. Thanks for posting the question!

Mr. I