How about this for a guideline, which Mega Man Legends made me think of for obvious reasons–if the month’s game is, say, part of a popular and relatively cohesive series that lots of people have played, and the entry in the series for the month is something people don’t necessarily have a desire to replay but makes them want to play other entries in the series, we can say that the thread can absolutely encompass discussion of other entries in the series.
Maybe, though, it’d make sense to also put on a 2 week moratorium on discussion and sharing of too much stuff from other entries in the series, so that the month’s thread is still primarily focused on the actual game chosen, at least when most people are playing it.
Perhaps I’m just being selfish, because as much as I love Mega Man Legends, I don’t have much of a desire to replay it! But it sounds like I might not be the only one who may want to play other entries in the series more than this entry in the series.