Now I have a co-developer! He will help designing several parts of the game on the paper, and he is also responsible for the design of the dynamic storyline for the game. He is sort if the idea guy for the project, he comes up with a lot of ideas, describes them. And it's up to me whether to implement those or not. He has already come up with a lot of ideas for the singleplayer part of the game, especially the RPG elements.
As a start for writing the game design document, I used a template. But I have found out that the template is not sufficient enough for my needs, so I have tweaked the template a lot to fit my needs, so now there are just a minimum of the original template left.