I would suggest multiple fortresses in a world, with an ultra-difficult one at the center that ANYONE near the center can attack, proven atleast 3/4s of the fortresses are destroyed. Each fortress is sheilded by multiple command hubs, same procedure as in a normal world. The level of the ultra fortress should be about lv. 90, and the max level for players should be near that. Multiple alliances can attack the ultra-fortress as if they are one big alliance, maybe like 200.
This however is only a suggestion for the feircest players, and from start to end game should take more than a year.
