First off, I want to say I've never used a design document before
Now, what I think:
It looks really great, and it could be useful.
One thing I don't like is that it seems very focused on jus strategy games. It doesn't look like it would be much use in a sports/pet game, and possibly useful in an RPG. There's just a lot of elements that are typical of strategy games but not other genres, such as construction, research, buildings.
Maybe you could change it into something more general, and give those specifics as examples. (For example, Personal/Player Development, which could refer to research in a strategy game, and stat training in an RPG)
I don't think I would use this design document the way you intended because it's the kind of stuff I just write down in notepad as I think of ideas, but maybe I'm just a lazy planner
I'd use it more of as a checklist for what elements I might want in a game.
The stuff that I really think of ahead is the database structure and relations, because once the game is made and being played, it's hard to change, whereas code changes and ideas I can implement much later on, and easier than database changes.
