Heh, if I knew the answer to this one, I might actually get a project started
Seriously though, the first thing I tend to do is just write down what I want it to do, I've had some ideas that are great in theory, but when it comes to writing down how things would work in practice, I suddenly find brick walls that I never knew were there.
With regards to similar projects, it's that thin line between inspiration and emulation, as it were, some methodologies in games make so much sense in certain circumstances that it is almost impossible not
to use something similar, the hard part is implementing it in a way that enhances or expands on the original implementation.
Also, personal competence comes into it in my case. I'd love to write a computer game, but if I did, it would be 2D Top-Down (or possibly isometric) graphics, because (a) I lack the mathematical and technical knowledge to go 3D and (b) I personally would rather concentrate my energies on coding a game engine than a graphics one, so the less effort I need to put into one thing, the more I have spare to put into another.
The hard part is always trying to find a compromise between my vision and my ability