I think we should start without waiting for every faction to be filled. The last BC was not full when we began, and only filled up as the game progressed. So long as the big ones are taken, there shouldn't be any issues of it being fair/unfair for some players by having an AI faction on their border. Remember, with BC, AI factions will get big bonuses - like the garrison scripts and having massive armies spawn at will - that will make fighting them no less difficult than fighting a player.