Just think about stuff, try and find something interesting and see if any gameplay related connections can be made. Sleep on it and see if anything good comes out of it when you wake up in the morning. At which point you might be awake, sober and better able to judge whether anything worthwhile came out of your ramblings.
Tell your buddies about it and if they are as enthused as you, they might throw some other angles at you which establish an idea with broader appeal.
Tell your buddies about it and if they are as enthused as you, they might throw some other angles at you which establish an idea with broader appeal.