I normally don't have much good to say about FFXI, but the most memorable fight I saw in that game was the fight for Outpost warp points. A giant Treant wandered around with millions upon millions of hit points that was meant to be killed over the course of a week. People threw themselves at the tree constantly, getting killed and warping back to have another go at it. Of course, the reward was incredible and it didn't cause an XP penalty, so people had good reason to make the attempt.

Perhaps in this case, the world boss could teleport to different zones (for a truly world-spanning challenge) over the course of its timer, and rather than being a straight level 50+ enemy, it could take similar damage from both level 1's and level 50's alike while dishing out damage proportional to the player's health.