I think it's pretty clear that the qualities needed by a video game to be valid are that it should have the same narrative for every user, just as other stories in all other media do. Since my original suggestion has been rather strongly opposed, my new suggestion is this:
Declare all video games invalid. The whole point of video games is to immerse oneself in the story. That means every user can play however he/she wants. This includes choosing characters, choosing paths, choosing what tasks to do or not do, choosing what order to do the tasks in, choosing who to kill, etc. etc. etc. etc. This means that video games intrinsically don't have one narrative for all users and that they therefore fail Tardis:Valid sources for the same reasons that stage plays and "choose your ending" stories fail.
Many of the arguments listed above (and in other threads) for keeping certain video games valid tend to ignore the issue of having the same narrative for all users. But like it or not, it is one of the rules for determining valid sources.