Well, currently we're okay with strategy guides, art books, plush toys of various sizes and dimensions, calendars, figurines, preorder items, and soundtrack CDs.
In my very personal opinion, fiction books (novelizations or original stories based on games), video game code books, comics in single issue or collected formats (adaptations [IDW's Metal Gear comics], tie-in stories, or original stories based on the characters [Valiant's Nintendo Comics System series]), movies based on video games (DVD / Blu-Ray / VHS, and other formats), strategy guides on video (like GamePRO's stuff), speed play videos (like those released in Japan), DVD games, Gamest videos (tapes or laserdiscs that showed combos and other information on games), books that deal with video games as a subject (the history of the medium, etc.), artbooks... should ALL be allowed.
But, I'm just one guy and that's not enough to institute policy.