Minor detail, but they would need to implement where each server is fighting its own boss, but the damage is linked to a global HP pool and global leaderboard. If ALL players were actually fighting the same boss and able to see each other, then it would crash the servers instantly. Also, you wouldn't want cross-server PVP going on with older server players farming newer servers players.

Just a minor implementation detail, but throwing it out there since chances are if the devs do decide to implement it, they will probably do it wrong somehow.