I was thinking with Assassin's Creed Rogue coming out soon why don't you rename Assassin's Creed Black Flag to Assassin's Creed Series?
It seems to be a deep independence and easy reasons as to why it may need to stay separate. Well first off I know I for sure wouldn't want to go through a generalized forum looking for a certain game types threads. Especially mod tools and all of that sorts. And another reason is it would be best to wait to see if Assassins Creed Unity is even popular enough to add a forum for it. If that's the case i'm sure there will be one added just not a merged one.