Buy yourself a bunch of paper and a good pen.
Then just write down everything which comes in your mind.
Drawings, characters, sory, level layout.
Then when you have a lots of written down ideas you look trough them and pick parts of it. Let you imagination fly as high as it want.
Dont think "what is possible? What is fun? what is cool?"
Just write down everything your brain comes up with
(I have about 70 A4 sheets full of stuff, everything from models to stories)
And dont even touch FPSC untill you know how you game will begin, twist and end.