I'm wondering if this applies to games that have been cancelled. Since the game doesn't actually exist, its my thinking that it should not have any concepts or characters associated with it, because the game doesn't exist.
The game existed at some point in development. If a dev said the game would contain X, or there was a video released clearly showing X, or a leaked prototype cartridge had X in it, I would think that's sufficient to assert that X would have been in the game, unless we know it was later removed. But if you don't know that it would have been in there, don't add it. Giant Bomb can be a useful resource for cancelled/vapourware games, as long as we don't make assumptions about the unknowns.