The actual rules of the Great Game (castling, etc) can be used unchanged, since they had no influence on the mechanics of 3e that needed to be replicated in 4e. It was merely a fluff explanation to why some dragons could cast spells and why others couldn't. Since every dragon now can't cast spells per se, Xorvintaal becomes a campaign option.So there are like no plans to actually make rules involving xorvintaal