Hi there! Not only that I am a Game Designer but I am also a Game Developer. I can provide all three major components of a Game Design: Art Bible, Technical Design, and Game Design. I can also provide the project timeline for management and estimates.
I plan to design the game first without any consideration to art, and programming side just to let the ideas flow. Then get to the art bible, just collecting art styles and the whole feel of the game, and finally the technical document where we will decide what to chop off on our game design (whether it's not possible, un-optimized, redundant).
After doing these three documents, we will now make a mock-up for the game, and now straight to timeline and estimates.
Please message me if you have any questions or so.