I am creating a website, where users are able to create guides for a video game. Since there is/could be a strict structure for the writing of the guide, I am not really sure if I actually should pretend a structure for the sections that should be filled out. Should I rather make a link where the recommended structure of a guide is shown and let the user eventually do what he wants to do, or shall I actually pretend a structure for the user so the guide follows a certain quality line?
Since you're talking about a competitor to GameFAQs, you should be looking at their model in great depth.
As I reminisce about when GameFAQs actually started (I do actually remember finding it in the original Yahoo! listing, back when the engine was moderated by real people), the number of guides I've used there, and the few guides I've written for it -- the site would be a strong case for not restricting structure. The site has existed largely untouched for a long time!
The success in its simplicity is similar to Craig's List's continued popularity. It isn't necessarily pretty, but I can very easily get the information I'm looking for and get out.
Street Fighter does not demand the same structure as Final Fantasy, or as Mass Effect, or another genre. Even within genre, different types of guides do better in different formats. Is this guide a walkthrough, a super-move cheat sheet, or is it about solving the complex mini-games within the larger gameplay? Different scenarios will not fit the same style.
You are also trying to create a site that is driven by community contributions... at least, I'm assuming you're not sending your users a check when write a guide. So, as an author, if I want to contribute something and you put too many restrictions on my "creative genius" I'll probably just go somewhere else (GameFAQs, probably).