I've noticed a few different questions recently that were better asked on the Game Development Stack Exchange site. Considering how closely the two sites are related, I would assume that such occurrences would only increase as site traffic increased. I think it'd be a good idea to add the Game Development SE site to our list of "off topic destination candidates". Currently, the only site there is Meta @ Gaming (see below).
In addition, I've seen one or two questions that were good candidates for WebApps or SuperUser. The frequency is so intermittent that I don't think we need them on this list at the moment, but if we're looking to fill out the list, those would be reasonable candidates.
Regardless of that, I firmly believe the Game Development SE would be a good fit for this list.