Answer some of these then add anything or explain why you chose what.
What elements in a FPS do you love?
What have you not yet seen?
What things from other games were good ideas, but could be done better?
What are the weapons/grenades?
What are the funnest elements?
What would make competitive and casual gamers happy?
What might a new story be about?
What is the perfect way to make maps?
What medals would you add/remove?
How would you make the physics work?
What would be the mood of the game?
What would be the dynamic colors?
How would be the online setup?
What would the vehicles be like, if any?
What gametypes would be included?
Would you include RPG/other genre elements?
What would be the setting?
What different kind of modes would there be?(theater, forge, create a map,)
Would you incorporate a clan type setup?
Would you include upgradable weapons?(CoD4 style or other)
Would you have different species to choose from to effect gameplay?(Shadowrun)
Would you have equipment, what types?(example:Jetpacks, Grapple)
Anything else?