You can probably turn an instanced variant of a particular zone into an all-out siege that where one must fight to recapture tactically advantageous vantage points and aetherytes while requiring a large part of the relevant active playerbase (e.g. those with level 70s online atm, scaled to a certain portion and then increasing difficulty and reward with participation in excess of that minimum/excess portion), but that'd be a content type in and off itself, not a world-wide shift of any sort.